I've not played through a Promethean so I can't offer one way or another there. But I am a happy BG250 owner. I've found the combo to be very versatile and more than loud enough with a variety of 5 stringers over a loud drummer and 2 guitars. 90% of the time, I load a SpectraComp in the Toneprint section and that's all I take to gigs. A combination of the SpectraComp and the tube drive is all I usually need to get what I want, especially with the footswitch. It's also great that both of those can be dialed in and out, instead of just being an on/off switch. Having an onboard tuner is heaven.
One thing, if it matters to you. The BG250 is ported, the Promethean is sealed. If you have a big preference on that, then that might be a strong factor.