It's 100% personal preference. I've tried .5, .7 and .38 and I prefer .38.
I own this pen (use it daily) and find that because it's a heavy pen and because I tend to write with a heavy hand, anything thicker than the .38 and the line is too bold for my tastes.
I'd say get the .5 and try the .7 (.7 refills are much easier to come by in your local office supply/convenience stores).
.38 G2 is absolutely fantastic in this pen. Makes a fine line, does not glob, is a fairly smooth glider. My favorite rolling ball cartridge of all time. The fact it fits this pen is why I bought one.
Get the one you think you'll like and if you end up not liking it, buy a thinner or thicker refill.
I didn't think I'd like this pen as much as I do but it's a good one! Worth it no matter which one you choose.