5310 is slimmer, has bigger and better display, music keys, 3.5mm jack, stereo speakers, A2DP. W810i on the other hand has better camera and comes with better headset. If you don't mind the comparatively lower quality shots of 5310 and can spend some money on a good headphone like Creative EP-630 (about 800 bucks) then i'll recommend 5310. Its advantages outweigh those of W810i.