If you need to go for a Camera, then K750i. Its a very good handset, But the disadvantage is it not a Smart Phone!
Nokia 6630 is a very nice phone and with the smart Symbian, you can install various applications (Both Java as well as SIS).
Its very difficult to get a Java Application for SE K750i as most of the Mobile Applications are in SIS. If you install / upload a theme to K750i, only the background will change and not the icons.
My friend asks me that whats difference is with a J300i software to K750i, it seems the same. With large amount of money, they have given Bluetooth, Camera and FM.
Now days FM comes with both from Best to worst hands-frees. In K750i, you can only hear FM if you connect Hands-free which is a disadvantage after giving so much money.
I totally agree with prankie and even I pick Nokia 6600 over SE K750i.
If you need only camera, then go to SE 750i. If you want other features then N6630 is recommended.
I have SE J300i and N6630 and happy with them.