I had similiar dilema, between U600 and K810i. My bro had a U600 so messed with that for a while, nice phone, but the camera didnt seem as good as the K810i so I went with the SE.
The camera is really good, the flash is like a real flash, it can take pics in the dark or when in low light.
Im used to a smartphone so was worried about getting a non smartphone, tbh though in the end you only ever end up using it as a phone anyway.
to me the biggest thing I also wanted out of the phone is to have a decent camera for those never miss shots with the kids, so I got the K810i
I have to say though Ive had it 2 months now, and the bottom off the keypad is a little loose already, I have contected SE direct and they saids send it back, I'll wait a bit longer and send it to them direct I think.
the U600 is a nice phone though, nice screen, you get used to the heat buttons.
hope this helps