a single speed can coast, whereas fixed, the pedals are constantly in motion as the wheels turn.

a bike with vertical dropouts is incredibly difficult to convert to ss/fg without an eno eccentric hub, so i won't recommend just avoiding that altogether, especially since you're on a budget.

here's a breakdown of the different parts you would need to convert the bike you linked to fixed gear:

new rear wheel: probably about $100. (if you're willing to mailorder, you can get a very nice wheel SET for 135)
cog: $20
lockring: $5
chain: $10
probably a new bottom bracket (i can't tell from the picture): $20
new crank set: $40
chainring: $20

so you're probably looking at about 200-250 dollars in addition to the cost of the frame, and you might want to replace things like tires and tubes, re-wrap the bars, etc.

no matter what, don't let anyone talk you into a "suicide hub" - if you go fixed, use a real track hub! sure, it's cheap, but it'll cause problems long-term.

if you wanted to do single-speed instead of fixed, you could conceivably get set up for about 150 dollars with a very solid setup. try someone's bike out to see if fixed is for you.

of course, used parts make everything cheaper, too.

did i answer your question?

oh, and i asked about school since i go to bu - always good to meet more bostonians!

EDIT: i didn't see that second bike you linked to - i can't really tell from the pictures, but it looks like it would be ok. that's the going rate here in boston (i've sold a few similar bikes for similar prices) but to be perfectly honest, it's not WORTH it... hold off and find a bike to convert - you'll learn how your bike works, so you'll be able to fix it when (and i do mean when, not if) it breaks. i mean, you said you're on a budget and shop repairs = $$$