The K750i is superior in pretty much every respect. It can expand the memory with Memory Stick Duo's, making it genuinely useful as an MP3 player, the radio has RDS, there's a fair few nice new bits of functionality, the camera is far, far, far superior (2 mp and has proper autofocus and a superior lens) and Java performance is significantly improved.
Certainly, given you can get the k750i for free on O2 200, I can't see any reason why you'd buy a k700 anymore. I love mine to bits.
Phazer