I've got a 6680 on Orange (cost £95 on PAYG), and my mate has a k750. I'd have the 6680 over the k750 because the 6680 runs on a Symbian operating system as previously mentioned, whereas the k750 only runs java software. The software for symbian phones is a lot better than java, and also, the 6680 will run all the apps a k750 will run.
If you can afford one, get a 6680!
EDIT: Whoops, didn't see you'd already bought it.