Err 1st requirement was look/design and battery life. so K810 is more better?   Cos i saw there's a comparison photo between k770 and k810 which k770 look more better

http://publish.it168.com/2007/0928/20070928000902.shtml

K770, the 14-mm thin cameraphone features a 3.2 megapixel, autofocus camera with LED photo light, and 1.9-inch QVGA display.

K810, The 3.2 megapixel camera comes packed with features. Get sharp with auto focus. And don't let poor lighting worry you: use the Xenon flash.

iRonTech,
From the two review which i quote on the webside, i can tell u that the k770i camera features not as k810.
1)K770 use LED photo light to take photo when at night mode.While K810 use the Xenon Flash.
2)Screen size k810(2.0 inch) bigger than k770(1.9inch).
3)if k770 camera software feature not as k770, u can try mod the hp.

For answer ur question 2, i would like to suggest u go for k850.
1)screen size for k770 1.9 inch, while k850 with 2.2 inch.
2)K770 use LED photo light to take photo when at night mode.While K850 use the Xenon Flash.
3)k850 network support HSDPA and EDGE. on the other hand k770 cannot. K770 support UMTS and gprs network.
4)K850  - 5 megapixel camera with auto focus and Xenon flash. K770-3.2 megapixel camera with LED photo light only.

ps: i do not go against anyone here.

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Added on March 17, 2008, 1:20 pm
squareballs,
U600 U700 and E900 using HEAT SENSITIVE BUTTONS not using touch screen.
1)U700 doesnt have fm radio , while U600 have.
2)Battery life for stand by time and talk time much less than the SonyErricson model.
3)U700 more expensive than 5610, K810 and U600.