If I were you, I'd go for a 48/19 ratio. I think you'll find that 48/20 feels nice coming out of red lights and on gentle uphill slopes, but then it starts to be frustratingly slow on the descents. I ran 32/13 (somewhere around 65") on my first fixed gear, and that ended up being a good way to learn to really spin, but I've since moved up to 46/17 with a 14t cog on the flop side, originally put there because I meant to get to the track in San Jose, but now it's mostly just for ****s and giggles.

I settled on 46/17 after going on a few more significant rides (10+ miles) with 32/13 and got really sick of spinning like a madman and not going fast for miles at a stretch. It also really made my knees sore, although that might have been the cumulative effect of fast spinning down the hill and mashing a slightly-too-high gear up it. I think if I were to do lots of longer rides on my fixed gear (mostly use the multi-geared bike for that), I would maybe go with 46/18 on one side for general riding and 46/15 on the other, since my track ends seem to be able to take up about a 3 tooth difference. As it is, though, in my day to day riding I don't have time to flip a wheel. 46/17 is a good all-around compromise ratio. The only time I flip the wheel around to the 14t is when I leave the Bike Co-op on the UCSC campus to go home. It's all downhill, and at 86" (with 23mm tires and 165mm cranks) it gets fast.