I recently bought both the Boyu and the UP because I've heard good things about both and wanted to compare for myself.
The UP looks a lot nicer and was easier to install because of the lock nuts, but I'm not actually sold on the build quality being better; the plastic seems quite brittle, whereas the Boyu is ugly but built like a tank.
The bubbles from the Boyu are quite large and the diffuser runs with almost no gas pressure at all; I reckon it could almost be used as an air stone with a standard air pump. The UP has bubbles so fine they're almost invisible, but I don't know if I got a slightly duff unit because mine needs 3 bar pressure before the gas will start to move.
Going back to what I said about the plastic seeming to be quite brittle, I can't say I'm too keen on pumping 3 bar pressure into plastic that could potentially burst under the strain. The UP diffuses the gas much more effectively, but when something is plumbed into a filter pipe you also need to be confident you won't end up with a flood.