I've tried both and definitely prefer the extra insurance of a fitted and cover (we use woolen longies in winter, covers in summer). Bitti boos are lovely.
Ecobubs pockets aren't bulky as of themselves, it totally depends on what you stuff them with, and I would say two bamboo trifolds definitely wouldn't give you a very big bum at all. They make great day nappies as well so if you decide not to use them at night you haven't wasted your money