first of all there is big price difference...w900i is for arnd 24-25K and w810i is for arnd 16K...so this shud be a big deciding factor
go for w900i if you want big screen and slider phone, apart form that it also has 3G but lacks edge connectivity, it also has bluetooth v2.0
go for w810i if you want a compact phone with light weight and gr8 durability and edge connectivity
as far as sound quality is concerned both are equally good and both offer same earphones and 512MB card...
camera quality is also same...
well these are the factors on which you can take ur decision...
as far as my suggestion is concerned i would say go for w810i coz its a more pratical option...if your main priority is music than definately go for w810i coz a portable music player should be as small and sleek as possible, moreover i wud not advice to pay extra 10K just for edge and big screen....
and if ur really willing to spend over 25K bucks than i wud suggest that go for N91 as it has better sound quality than w900i and plus it also has symbian and 4GB drive which will increase its longetivity....
but the bottom line is that we should not spend too much on phones like w900i or N91 as they are still evolving and within a gap of few months a much better model generally gets available...so think and spend carefully...
ENjoy...