K850i is an awful, unreliable phone that's widely considered to be Sony Ericssons poorest attempt in recent years. Please take my advice, as somebody who owned two that both broke, avoid them! If not my advice, just read through the threads about it on here complaining of faulty screens, touch sensitive keypads stopping working, resets, freezes etc etc etc
The camera on it doesn't live up to it's potential either, with the K800i/K810i often outperforming it!
Sony Ericsson are about to release the C902, which offers everything the K850i does but in a slimmer case with a better keypad and still has the 5MP camera
The W890i is a nice phone, but is more aimed at music then photography so if taking pics is important you'll be disappointed, as the camera isn't too good. Fine for viewing on the phone screen only, but that's about it sadly.
If you only get a new phone at the start or renewal of your contract, please don't rush out and get something you'll ultimately be disappointed with. Wait a while and get something that does what you want and does it well. Wait for the C902