Having not played BESM, but looking through the SRD of it a few times ...
BESM seems to have some complex elements that aren't really needed for an American supers game. Point buy seems to be a bit more complex, etc etc.
My group is getting a little more into M&M, with a bi-weekly Nocturnals game and now a "when we're bored" Teen Titans Animated Series game. We've had a turn-around in how much we like the system.
The combat system is very fast, very cinematic, and very superhero. Bad guys can get plastered in the first round, or a big brick can take EVERYTHING the team throws at them and then get crippled by his achilles heel.
The Damage Save system speeds combat up quite a bit. In a combat-heavy genre, that's a plus. Our nocturnals game had three combats in a single short evening, interwoven with the non-combat sections. It doesn't increase combat-heaviness, but makes combat take along about the same amount of time as NON-Combat areas of the game. I've always hated how RPing, skill checks, and other story-advancing elements of the game move quickly, and combat bogs down and takes an hour and a half to resolve 36 seconds of game-world-time.
Hit, Save, Hit, Save, Hit, Down ... yer done! Takes longer to describe the action than resolve it. Booya.
--fje