There is not much differences between Palm pre and iPhone 3G S in mode of price.You can get both at the same price but with different spec.

Based on Memory:
You can get a 16 GB iPhone 3G S for the price of 8GB Palm Pre
Camera:
Palm Pre lags here when we comes to camera.Its doesn't have the video recording feature while iPhone 3G S has it with an added feaure of Video Editting you can tring your movie jus with a single tap.You can share the movie/videos to the web in a fractioon of touch.Only advantage of Palm Pre's camera is its flash.But no one will fall for that flash.
Battery:
Palm Pre has a removable battery while iPhone 3G S doesn't have one but iPhone 3G S claims to be long life as apple improved thier battery quality while compared to that of iPhone 3G.So a removable battery doesn't matter a lot but still apple needs to design a removable battery to catch more market.:)

Iphone 3G S is more faster while compared to that of Palm pre as its processor make it work faster.Apple's advanced technology is moving beyond its limit with this kind of processor designs