I listen to mostly Rock and Metal and I have a relatively small ear size so comfort will not be a problem.
Well, to be honest, with a huge price drop like that, I would definitely grab the Momentum right then. Price drops don't mean they're not good; price follows the flow of supply and demand. Apparently demand for the Momentum isn't very high, which should explain the price drop. Were I you, I would grab the Momentum while they're cheap.
From your musical preferences, I don't think either the M100 or the Momentum will give you maximum enjoyment. Best case scenario is to listen to them for yourself and make your judgements.
Also, you can edit your posts even after you posted it.