If you think the N3 might be too big to pocket and can wait for the S5 to arrive, that doesn't sound like a bad idea to me. From what I've read the specs should be very similar or slightly better. Plus there are rumors of a 4K, non-AMOLED screen, an iris scanner and a different GUI which may or may not still be Touch-Whiz. Really hard to tell until the official announcement, but it could be a very compelling device.
All that said, I think the N3 is, overall, the best phone out there at the moment. It's plenty pocketable (depends on your pockets) and the t-mobile version is known to have an unlocked boot-loader. That plus tbe CPU, screen & microSD slot mean that, unless the S5 makes sandwiches, I don't expect to feel any buyers remorse when it releases. But I do love sandwiches.
tl;dr IMHO If you can, wait until the rumored S5 announcement in Feb and decide then. If not, get the N3 now. I like sandwiches.