captivate, runs android as it's os which is very good. the materials made out of it is durable. the specs inside the actual phone and far more better than the iphone as well as the focus (the screen on focus is the same as the captivate though). overall screen size is bigger.. no one wants a small screen.. and no more pixels doesn't make the screen bigger.
iphone remains the same from the previous generation, only new stuff would be hardware, some software stuff that it should of gotten 20 years ago, and new body design.
focus would be betting since it's a "new" platform, so you wouldn't really know the future of the windows 7 for phones.. but it looks promising. you can always be a early adopter

i say go for the captivate. i myself have the vibrant.