Ok, so I'm planning to buy a IRO Mark V, but can't decide on a fixed gear or a single speed.

FIXIE
Pros:
-Total awesomeness
-Skid till tires are burning
Cons:
-Going downhill (or just let my legs hang?)

SINGLE SPEED
Pros:
-Coasting
-Speeding...downhill
Cons:
-No skidding!
-even "Fixie" sounds better


HEELLP!

oh and what differences do the cranks and cogs make?

fixed gear with a front brake

Fixies and single-speeds look similar but they're very different. Tell us what you're after.

I think it's unfortunate that they're lumped together. I understand why, but they're more different than people seem to think. They're different ways to ride!

And can't you buy it with a flip-flop wheel so you can change your mind after purchase?

Will this be your only bike? What kind of riding do you expect to do? What kind of riding have you been doing recently? What's the terrain like where you are? What kind of condition are you in? Why are you considering SS or FG?

Get a flip flop hub and decide for yourself. There is more to fixed gear than burning skids.

Coaster brake.

Pros:
-Total awesomeness
-Skid till tires are burning
-Coasting
-Speeding...downhill
Cons:
-None

+ 1000000 on the flip-flop hub... that's why it exists (and is pretty popular)

I actually bought a Specialized Allez a week ago, but turns out it was too big, so I have to sell that.Then I started looking again and discovered single/fixie and totally want one! So this next bike I buy will be the only decent bike I will use, mostly to commute. In fact, it would be my first serious road bike. I live in San Francisco, although I probably won't be biking across the whole city often. But when I bike to school, there are some steep hills involved (down-to; up-from), although I don't really mind that. I believe that SS and FG are easier to maintain? which is already a plus because they look awesome! (my last Allez shipped with a bent derailleur, and had to get it fixed)
I'm leaning a bit towards SS because I may not want to be pedaling 100% of the time (so coast) but fixies seem pretty awesome!

Excuse my newbness, but what exactly is a coaster brake?

and still wondering what difference sizes of cogs/cranks make?

Fixed gear / single speed bikes aren't really a good choice for commutes with steep/long hills.

Yeah, I know, but I'm also planning to keep this bike for college in a year, which would probably be ok for in and out of campus.

Get what you want. Get a flip-flop hub which will allow you to switch back and forth and decide for yourself. Talk to someone at a bike shop about gearing. The problem you might have is that the easier it is for you to get up the hills the harder it's going to be go fast down them...and if you're on a fixed gear you might rattle your leg off at the hip.

Defintiely get a flip flop. I tried going FG at first by practicing in a lot at night and I realized it is difficult to get used to so now I just ride SS and couldn't be happier.

htfu and learn to spin down a hill

Ok, so do people really type like this or is it just on this forum? Please tell me it's just here as I have seen others start a thread with those words.

Skidding a coaster brake while bombing down a hill seems a little dangerous. Brakeless that is.

What do you mean? It's just a common way of expressing the same idea. Do you hear people say: "How was your day" or "What is your opinion on the progress of your particular set of given events in the 24 hr time frame"? No one types like that.
I definitely hear people say the same things more than once. It's just a typical way of colloquial communication. Now, by sheer coincidence, you have come by more than one person who has decided to purchase an IRO Mark V bicycle and who, had the slightest indecision in their mind about a SS or FG mechanism, and who has decided to consider the experience and insight of fellow cyclists. So, I assure you I am neither the creator of the previous thread with the same combination of words, nor have I ever inquired anyone about this matter.
Does that answer your question?

You're going to school in San Francisco? So is my daughter. She's at the Art Institute. Where are you?

I saw a lot of folks riding SS and FG bikes there and think they're crazy, especially those riding fixies without brakes. The hills there are breathtakingly steep and difficult. If you manage to ride up and down those hills on either type of bike, you'll be in excellent shape.

Everyone else, if you don't have any idea what hills are like in San Francisco, you might have trouble imagining them. The pictures don't depict them adequately!

I have an IRO Mark V with a flip flop hub, and its definitely worth it.
Somedays you feel like coasting, somedays you feel like skidding.
Although this has happened to me and most other people I know who got a flip flop hub, after I got used to riding fixed, I havent switched back.

Oh, here are responses to your other questions.

A coaster brake is a wheel with a coaster brake hub. It can coast like a regular bike, and when you pedal backwards, it actuates a brake inside the hub. The harder you turn the pedals backwards, the more braking action you get. There is a side benefit in this where by putting your weight backward on the rear pedal, you move your body weight over the rear wheel, helping the braking action along.

Size of chainring (front sprocket) and cog (rear sprocket) is very significant. The greater the ratio between the former and the latter, the higher the "gear" as we say. High gears help you go fast but are harder to accelerate, decelerate and climb hills. Low gears are good for acceleration, deceleration, and hill climbing but limit your top speed.

On a fixed gear bike, I generally recommend a gear (or really effective rear wheel size) of 65 to 72 inches. Calculate gear inches this way:

G = F / R * 27

where

G is gear inches
F is number of teeth on front sprocket
R is number of teeth on rear sprocket
27 is approximate size, in inches, of your wheel

Since you have crazy hills where you are, you may want something smaller than 65 inches.

The hills here are definitely crazy steep! Thank god I don't have to bike through the really steep ones because I don't really live live in SF anymore. I grew up in SF, but we moved to Daly City, but technically I "live" in SF (says the school district, heh). I still go to school in San Francisco though, and its about a 10 minute drive. (So you can probably guess where my high school is)
Biking to school will be a breeze, but heading back uphill would be a different story.

I think I am going to get a flip-flop hub, but does IRO sell those? or do I have to get them someplace else? Thanks.

fixie

Not really. I don't see people typing sentences that start with OK anywhere except for here. Maybe I am not hip enough. I just ride a Kilo.

Skidding anything down a hill is dangerous.
You aren't brakeless if you have a coaster brake.

Uh, so like, um, how are 'skidz' a pro to having a bike? Jesus dude, get a ****ing brake on your bike and don't be such a dumbass. I know you're excited, but a brake will probably save you a lot of pain, and possibly your life. And skidding through tires might be fun on the first day you learn it, but then when you have to keep buying new tires it gets a little old. And 'fixies' are only 'totally awesome' if the person riding it is 'totally capable of bringing out the total awesomeness of totally awesome 'fixie''.

This isn't even close to a Pro. Eventually coasting will feel broken. I don't just say that as a 'LOOK HOW KEWL I AM ON M FIXXIE YOUR A POOSPOOS!' way, it's just the truth.

And if you ride a single speed freewheel (or any freewheel), be sure you have two brakes.