If it's between w810i/k610i definitely the w810i.
Here are some bad points from both phones.
K610i- Its probably like the K550i, when you take a picture, you cannot zoom in when in 2MP/1MP mode, only in VGA (640×480) (w810i does not have this problem). You can however, zoom in after picture is taken and zoom in during video recording.
W810i- Viewing videos on fullscreen is really pixellated. It has a screen res. of 176x220 but can only play videos of 176x144. (this also occurs in K610i)
If you want a walkman phone get the W810i, stay away from w850i if you need decent camera and don't mind keypad cracking.
If you need a Kamera phone, look at the k790i/k800i/k810
EDIT: K610i cannot zoom in during 2MP/1MP mode.