I'm going to say everyone above is spot on. The difference in range between two equal guns one at 350fps and one at 400fps is not a whole heck of allot. If I had to pick a single gun it would be 350fps since it gives that flexibility. Indoors it's really difficult to run a 400fps gun with MED's enough so that it's not recommended in the rules for that reason. If your playing outdoors it's much easier and the engagements tend to be much further out. But we're talking about around 60 yards of effective range at 350fps, and somewhere around 70 yards with 400fps all things being equal. Now my numbers might be a little off, some guns just shoot a little further than others but in my collection of guns I've got everything from stock TM's to upgraded guns shooting 550fps, and really until you reach the long side of the FPS range I don't see much difference, and even then it's not massive but you get the ability to run heavier ammo and stretch that range further.
So if you've got one gun go with 350fps, if you want more, go out and pick up another gun to upgrade;) That's always good advice since upgraded guns past 350fps tend to lower reliability as such you'll always likely have a functional gun even if your upgraded gun fails.
Luke