overall, k550i = k750i + edge + cybershot brand name + memory stick micro (instead of memory stick pro duo as used in k750i)..................................................(eqn 1)
however do check out the keypad ergonomics on it. rite now memory stick micro is the costliest type of memory card in the market.
but if u want great music out of the box then buy w700i.
in terms of cam: w700i = k750i - autofocus..................................................(eqn2)
in terms of music: w700i = k750i + walkman brand (player and megabass) + hpm70 passive noise cancellation earbuds + 256mb memory stick pro duo..................................................(eqn3)
to compare between k550i and w700i, substitute for k750i in eqn2 and eqn3 from eqn1 in terms of k550i..... man! i miss school dayz.. used to these substitutions a lot!
ps: if u are adventerous enough u can incorporate the walkman features (player + megabass) in k750i by hacking. but u'll still need to purchase hpm70 and larger memory card separetely!