Which you choose will be affected by your physiology and the terrain that you'll mostly be using the machine on. If you've a smaller build, the more natural climber or the terrain is more hilly, you'll probably tend towards a lighter gear.
I tend towards the fast twitch end of the spectrum so I generally ride a heavier gear, but my terrain is pretty flat with only the occasional hill, so those two things match quite nicely.
If you're not sure, then I would suggest that you start light and get heavier - it's generally easier to spin your cadence up then to hammer your knees crunching along in a gear that's too low.
What you can do is have two cogs on the same rear hub, with different rings on so you could start on something quite light and then switch it to something heavier, e.g assuming you've something like a 46 on the front, you might have an 18 on one side and a 17 (or 16) on the rear.
If you do think that you're likely to end up with a heavier gearing then you would probably want to go with a larger front cog, e.g. a 48. It's a lot easier to change the rear than the front.
With due respect to @lantius I wouldn't start with a single speed and gravitate to a fixed, they are quite a different experience. If you want to ride fixed, just ride fixed. (To be honest, I've never really seen the point of singlespeeds, but that's not really an answer to a question, just an opinion!)