I did indeed decide to get the Captivate rather than the iPhone 4. Right now I'm a bit concerned about call quality (that's very important to me since I use it for work), and it would be nearly impossible to get a replacement promptly if there were any problems given the lack of inventory. I confess I'm bored a little bit with iOS too, though not enough to give up my iPhone 3GS . Personally, I'm not in love with iOS 4's multi-tasking, but it's better than nothing. BTW, the CPU/GPUs are comparable.
I go through a lot of phones every year, but this time I was actually eligible for a contract upgrade.
As to which you should get, that depends on how much you like the iPhone 4 and how well it's working for you. There's always a tempting competitor out there, but if you're happy enough with the iPhone, there's nothing wrong with keeping it. But if you're really bored with the interface and OS and love those widgets, well, there's a reason to switch.
-------------------- Lisa Gade Editor in Chief, MobileTechReview