I made the move from pre to EVO a few weeks ago. Things that stand out for me:
1. Pre's multi-tasking (cards) is much easier/useful than Android's. I would say, WebOS ui in general is better.
2. EVO/Android, has lots to play with--you won't get bored.
3. Pre's battery lasts longer than EVO.
4. EVO is much faster.
5. EVO is more stable.
6. Less hacks needed to make your EVO or android device do what you want it to.
7. I prefer Pre's hardware keyboard to EVO's soft keyboard. But that is the price you pay for a much more usable/large screen.
8. Mail application on EVO/Android does not support Exchange Tasks. 3rd party app I tried supports Tasks but doesn't support additional POP accounts. Likewise "Tentative" option is not supported in Exchange meeting invitations. If you don't use exchange, then don't worry.
9. Way more apps available for Android. And, I think WebOS will never catch up/catch on, unfortunately.
10. EVO's camera is way better than Pre's although flash is too strong for close-ups.
11. Speaking of Flash...Android has Adobe Flash support and pre doesn't. Big deal for me.