Doesn't matter. Protein is protein. I have tried both and personally, the only difference is flavor. The standard chocolate for MTS is great, but so is the banana ON, Mocha ON, Strawberry ON, and Vanilla ON. Whichever's cheaper is the one I would go with.
Both are top notch quality wise, ON always seems to lack in taste and tastes very watered down. If I were you I would give MTS whey a shot, but I can't speak on flavor for that because I have not tried it.