I have been studying everything about the EVO and the next gen iPhone, too.. I am very tempted to jump ship and go to Sprint for the EVO for these reasons:
The 4G. AT&T doesn't have it. The Evo will, although I'm not sure if my house will receive it. I checked the sprint coverage map and my house is right on the very edge!
That giant screen on the Evo! The next iPhone has the same sized screen as the previous versions from what I can tell, although it should be a much greater resolution which is about time. But that doesn't make up for the size. The evo will have both a giant and HD screen.
The pricing is a little better for all that you get by getting the Evo on Sprint. I won't pay an extra $30 a month for the hotspot app though. I hope that there will be a way to get that done for free on Android though. I don't know.
The 8 mp camera on the evo is a very big bonus with HD video recording as well. The iPhone should record HD, too, though. The video recording from my current iPhone 3GS is excellent and it should only be better on the version. Both phones will have a front facing camera. Not sure if At&t can handle video chatting though, so Sprint's 4G should definitely have the advantage there. Both phones will have a flash, too.
Sprint everything plans with pretty much unlimited talk (by being able to talk to any other cell phones unlimited) is a huge bonus. My wife loves to talk and usually gets close to going over our allotted minutes on our iPhones.
Evo has a kickstand, iPhone won't.
Evo has an HDMI output, iPhone won't.
Evo has an SD card, iPhone won't.
Now I'm concerned about syncing to my iMac though, not sure that will be an issue though. Probably not. I know it won't be nearly as smooth as syncing my iPhone to my iMac though.
Oh yeah, how would I get my music from iTunes to the evo??
Anyway, many things to consider, plus the early termination fees for 2 iPhones on our family plan. And connection fees from sprint for 2 new Evo's. I wait until apple announces the next iPhone to make a decision. Sprint does give you 30 days to try it out and they will not charge you for anything if you decide to return it. Ok I'm done now. Thanks.