In many ways they are both extremely similar, we have even done many similar things to them. It's the finer things that make the difference.
I prefer harmonia, but I LIKE Bumblebee. If Harmonia didn't exist, I would be using it. If you want a kick butt rom, that is it. Obijohn has my total respect for what he has done. I prefer Harmonia because it's a stable blank canvas to do with what I please. It's the phone Virgin should have shipped. Then if people wanted, they could have added Bumblebee or CM7 for something above and beyond. I see Harmonia as the "generic rom" and Bumblebee and CM7 as specialist roms.
By the way Barfmog, Harmonia had Mvdalvik before Bumblebee. It was the first 2.2 OV rom to have it.
It goes back and forth, one rom gets something then the rest do.
Oh, as for CM7, it;s nice, but a bit overkill and buggy. Reborn, I love the looks, and it has some nice features, but it missing mvdalvik is a serious Achilles heal, but it is a nice rom. A good successor to Rodimus. There are other roms as well and all have their pros and cons, but these are the big 4.