K550i because its new so it has a lot new 'snazzy' features like track ID, which k750 does not have,it uses M2 instead of Pro duo because M2 is cheaper, sleeker,slimmer and the headfones can fit in your ear. Being a former K750i owner my self i did like it, but i prefare K550i....

P.S: k550i It can be flashed.