The only one of those I've played is GURPS, and that wasn't for supers. I've
heard that supers isn't one of the game's strong suits, whereas with HERO, it's perhaps
that game's strong
est suit. Not sure.
M&M, I would throroughly recommend. And now, there's the DCA RPG (which is, basically, M&M 3rd edition) which, to me, looks even better. Time will tell.
On the lighter end (rules-wise) there are such things as ICONS and - even lighter, I believe - BASH (Basic Action Super Heroes). They both seem quite neat as well, and have devoted fans, just as do all the others mentioned.
There are quickstart / intro / basic / "lite" versions of most of those rulesets available for free, IIRC. Certainly, that's true of M&M and GURPS. Plus, you can look at your friends' books for HERO and GURPS (the full versions thereof).
Generally, the best way is to give games a look for yourself. Even better, give them a try! But, I know, that requires a lot more investment, of time if nothing else. And players can get mipatient or cheesed off if GMs constantly switch systems.
So yeah, "look, don't touch" might be best, for starters.
edit: Oh, and the other thing is forums. Not that this one isn't great - it is, of course!

But the ones dedicated to each system (e.g., atomicthinktank for M&M) are
excellent for info, advice, and statted up characters of any kind you can dream of, just about. Which will give you a lot of insight into how the system works and even plays. Worth a visit, any or all.