Bellatrix would of learned Dark magic from voldemort

This is a tough pick, i think Minerva would probably win, while Bellatrix no doubt has a grasp of dark magic not only because of her association with Voldemort but also because of the family she grew up in, i think that Bellatrix is impulsive and often so enjoys the pain she is causing that she loses focus on the fights she is in, i think Minerva would out - think her simply by staying calm and thinking over her strategy carefully.
While we don't see a lot of Minerva dueling in HP we know that she is an accomplished and very powerful witch with masters in various subjects, her wisdom would also no doubt lend itself to winning this duel as well.

Hard

I'll just link you to a very heated debate I had about this; link

She is so much wiser, has more knowlage and is more skilled. And she is old and... powerful! ;)

McGonagall is stronger than we'll ever know.

I think if Bellatrix could be beaten by Molly Weasley, Minerva could beat her easily.

McGonagall seems like a really powerful witch.