I'm trying to decide whether or not to upgrade, as well. I have a 4S, and just recently became eligible for upgrade on AT&T. I'm not sure if I should upgrade to the 5 now, or wait for the 5S. I had the original on launch day, skipped the 3G, eligible for and got the 3GS, skipped the 4, eligible for and got the 4S. I'm someone who carries a ton of media on my phone, so I have a 64GB, that is maxed. I can wait for a 5S, but it probably isn't going to come out until Sept/Oct. If it comes out with 128GB, I'll be happy. And given the history of 3GS going to 32GB and 4S going to 128GB, the 5S might move up. The other potential upgrades probably aren't that big a deal to me. Obviously a form factor change would be cool, but I don't see it happening with this release.
So, I could upgrade to the 5 now, and essentially make myself eligible for when the 6 comes out, or soon after. Likewise, right now my 4S is only one generation old, so may have more resell value than it will have when the 5S comes out. If I wait for the 5S, it pretty much means I will miss the 6, which is when the next likely form factor change takes place. 😕 Decision, decisions ...