I completely disagree with a lot of the iphone biased statements on here. I had the Pre for 29 days and returned it (but I might go back once palm adds a few more features). Once you have 5 cards open on the pre, switching between them (with or without advanced gestures) is definitely faster than switching apps on the iphone 3g / 3gs. Now if you close apps on the pre and reopen every time, of course its slower!
In battery tests, the pre got longer talk time than the iphone 3g/3gs, but if you are on wifi the 3g / 3gs got longer time. It really comes down to how you use your phone.....if you are a power business user, using 3g talk, 3g data mostly, the pre and 3g/3gs all compare very similarly.....if you are talking about playing music and watching video's constantly, the iphone is definitely a better choice. Leaving the wrong apps open will affect battery life, so you have to have 1 brain cell to figure that out....if you can't, iphone is definitely a better choice.
The hardware on the pre definitely isn't as solid as the iphone, but you have to take into effect that the pre has a significant moving part, while the iphone, does not. I got a pretty solid pre and didn't have to replace it, but I do know that some of the build quality isn't great. I think some of the problem has to do with how people are opening the slider. If you push from the bottom, I think it creates problems with the sliding mechansim, if you push from the middle and pull down on the back side like the manual states, it works well.
I like both devices and it was hard giving up the pre and the money I would have saved with sprint, but a few exchange features on the iphone tugged me back to evil ATT and the iphone 3g/3gs.
I'm not going to provide biased information like other people, I think you should give both devices a try and see what works better for you. You should go into it knowing the things that are important for you....thats what works best. In the end, for me, it was still a really difficult decision because they are both great devices.