I think I paid ?25 for the RS-90 and ?31 for the Bittboy (well the Bittboy was a gift from Mrs. Lemmywinks so technically she paid for it!) so the difference isn't massive, the RS-90 was one of my most played handhelds last year along with the GPD Win. I love that little thing, it doesn't do anything particularly well but hopefully it can run CFW at some point, with the micro handhelds all I want is 8 bit console and handheld emulation really. I think if the Bittboy got SMS/GG emulation using the official firmware I would leave it like that, the UI is very nice.
There's still the resolution problem with the RS-90 though so maybe anything other than handheld roms will always look a bit borked, plus mine is a little worse for wear due to travelling in my work bag full time. I have a couple of dead pixels now and recently had to open it up to bend the speaker contacts as audio was cutting out.