Comparing your preference in authors could be a big selling point for you, too.
Steve Kenson is amazing. M&M is on my "to-buy" list simply because I love his writing.
BESM d20 has some great authors: Mark C MacKinnon is another one of my favorites, and Alejandro Melchor did the Shaman's book for Mongoose, I think. I don't recognize the other names as well, except for David Pulver, the man behind GURPS Vehicles (eh) and GURPS UltraTech (oof). His work on BESM is awesome - most especially the robust, effects-based d20Mecha system, which is absolutely amazing.
Anyways, if you're familiar with the "gaming style" of the various authors of either system, and know how their writing meshes with your own preferences, that might help you make a decison. Personally, I'd give M&M a shot simply based on how much I loved the Psychic's Handbook by Kenson, though I don't own it yet. BESM d20 has a lot of interesting features that I'm considering adopting, but I wouldn't use it entirely as-written for my next game. It might be fun to try for a one-shot though. d20Mecha, on the other hand, is abso-freakin-lutely amazing and I'd recommend it for any style or genre of play, and maybe even try to mess with it as vehicle system for non-d20 games.
So, anywho - BESM d20 could be used for a supers game, a modern game, a sci-fi game, a fantasy game, etc. The person who mention the spikey hairs, the funny colors, and the half-demon (which is a character that shows up in plenty of American and British comics...) seriously missed the mark. *However* - don't dismiss Mutants and Masterminds, as I love the main author, it's received rave reviews, and it's specifically designed for super-hero stuff.
Adios!
-George