Y i am suggesting k550i over k750i is coz :
k750i is a really old model nw..... doesnt looks much good n k550i is slimmer
K550i has A2DP , EDGE , Fast Proccy , newer interface
The diff in their cams isnt much...n as i hav said k550i's cam rocks....
Dunno abt the music quality of k750i ...so cant comment
The default headset u get is hpm -62....it lacks bass n music quality....
U can buy the inear Hpm - 70(it has a 3.5 mm jack) costing arnd 800 buck if u want a better musical experience.....
So choose nw ... as per the features u want...