singlespeed hub, or fixied gear hub W/ freewheel cog?
 

If you know you want a single speed, would it be best to get a singlespeed hub, or a Fixed Gear hub with a freewheel cog? Or is that basically the same thing?

I am concerned with the difference in weight, durability, longevity. Not necessarily in that order.

Some of your terms are a little confusing.

They make two types of flip/flop hubs, Fixed/Free and Fixed/Fixed. They also make just fixed hubs and just singlespeed hubs. It seems to be generally accepted that a fixed hub can (most of the time) accommodate a freewheel no problem, but depends on the thickness of the freewheel and the number of threads available before the lockring section.

However it does not go the other way around, you shouldn't put a fixed cog on a freewheel hub.

Obviously if you're worried about durability and longevity, it's in your best interest to go with what the hub is designed for, fixed cog on a fix hub, freewheel on a singlespeed hub.

I would guess that generally a one sided hub fixed cog (as opposed to a flip/flop) would be lighter than a free wheel set up, but weight is also dependent on material, flange height, and the number of pawls the freewheel has. The total weight can vary greatly depending on what components you choose.

Go fixed. You know you want to.

To be more clear, I am either going to build a fixed only hub and put a freewheel on it, or buy a freewheel only hub. I have no desire for a flip/flop. Just wondering which one is better. Having never owned a fixed hub or a freewheel cog, I don't know much about them.

In your case a fixed hub would be a pretty safe bet, you can use both fixed cogs and freewheels on them if you ever wanted to switch to fixed. But if you know you're going to run SS only, then a freewheel hub would be the best option. The only difference is the way they are threaded.

DuraAce track hubs only come in single sided fixed hubs I believe, so for the example given you don't have a choice, it's fixed hub or nothing.

Flip flop just means it's got threads on both sides. That's your best option because it allows you to try SS or FG or both AND it allows you to carry two cogs/freewheels whatever so you have a choice of gears, useful at least in the early days when you're trying to work out what sort of gear to use. Besides, the things are so readily available, you're actually reducing your choice of hubs by looking for something else.