The only two d20 supers game I've had the opportunity to look through that haven't been discussed already are Guardians of Order's "Silver Age Sentinels d20" and Green Ronin's "Mutants & Masterminds" (the playtest version, anyway). And of course my NDA with Green Ronin means that when I compare the two systems I really can't be too specific when it comes to M&M. But I'll try and hit the high points...
THE BOOKS: Both books are or will be hardcover, since M&M isn't in print yet. Unfortunately, SAS d20 is in B&W, while M&M will be full color. On a completely personal level, I'm one of artist Ed Northcott's biggest fans, so I like the art for SAS d20 better than what I've seen of M&M's so far. By contrast if the fast-play PDF for M&M is any indication of what the interior's going to look like, the color pages, the comic fonts, the sequential panels, etc. makes it look more like an actual comic book than SAS d20 does.
DICE: M&M uses 1d20 and that's it. SAS d20 uses all of the usual dice, and lots of 'em. I'll talk more about that later, but frankly I say the more dice the better!
STATS: In M&M, all six stats top out at 20 -- past that everything is a seperate super-power. In SAS d20 the stats progress normally past 20 just like they do in D&D, although they might operate a little differently (for instance in SAS you don't get automatic bonuses to hit from STR and DEX). So in M&M a strongman might have a STR 20 and 10 ranks in Super Strength, in SAS d20 that same strongman would have a STR of 70. Between the two systems, if your heroes are going to be fighting monsters from other d20 games, SAS d20 is gonna make things easier for you.
CLASSES: SAS d20 uses nine new character classes and lays points down on top of them, while M&M is classless and entirely point based. But to be fair, SAS d20 has an "Adventurer" class that is completely generic, which lets GMs use it as the only class in their game if they want -- making it effectively classless. Alternatively, I suppose you could also import classes from other games. In both systems the players can spend points to adjust stats, buy more HD, increase their BABs, etc., but since M&M is entirely classless you've got a little extra wiggle room there.
LEVELS: SAS d20 uses the standard D&D experience chart, M&M awards power points directly and determines your level by the number of points spent. In M&M most heroes start at 10th level, while SAS d20 characters typically start at 1st and have an ECL based on the number of points spent. And because SAS allows beginning characters to effectively buy more levels, in a 150 pt. campaign you might either have a 1st lvl hero with 150 pts to spend or a 10th lvl hero with 50 points to spend. I guess it's more semantics between the two games than anything else.
FEATS AND SKILLS: Anyone who's seen the SAS d20 smackdown in these forums before knows the deal -- M&M uses feats and skills pretty much identical to other d20 games (including a few new super-feats), while SAS doesn't have feats at all and uses a skill list identical to the one from its Tri-Stat games, including the specialization rules. Yet at the same time the way skills are acquired in SAS d20 is actually closer to D&D, because there are class skills, cross-class skills, ranks per level, and X number of skill points gained per level. M&M's are bought with power points and the only restrictions on them are the ranks. Me, I like SAS d20's method better but again that's because I prefer BESM and other Tri-Stat games over D&D, and I'm already used to the way they do things.
POWERS: Because SAS d20 doesn't have feats, anything that's out of the ordinary is considered a power and bought with points and ranks in it accordingly. This too is almost identical to the way things work in Tri-Stat, right down to the point values and the PMVs. M&M handles things a little bit more simply, with ranks, extras, and stunts that are bit closer to what you'd find in Marvel SAGA. M&M does include creation rules for adding completely new powers to the game, but SAS d20 doesn't outside of customizing powers like "Special Attack". My own thinking on this is that SAS d20 is enough like Tri-Stat that you really don't actually need rules showing you how to do this, especially with the PMVs.
COMBAT AND DAMAGE: M&M uses 1d20 and a completely unique set of rules for damage designed to simulate comics, complete with ranks in place of damage dice and a new "Damage" saving throw. If you've seen the fast-play PDF then you know what I mean. Likewise, you don't roll dice for weaponry, the ranks indicate the penalty to the Damage save. SAS d20 uses HP, standard die values for weapons, etc., that make it 100% compatible with D&D and other d20 games right out of the box. The only tweaking you'd need to do is armor -- SAS d20 treats physical armor as damage resistance (thank goodness). Now M&M has variant HP rules as well, but I've got some issues with them that I really can't address here. Another plus for SAS d20 if you like the Tri-Stat combat rules is that it introduces a number of them to the d20 System, such as called shots, Shock Value, etc.
CAMPAIGN SETTING: With SAS d20 you're getting a ton of information about GoO's "Empire City" world, complete with NPCs, organizations, and Empire City itself (an alternate version of NYC). M&M has a brief intro adventure set in Green Ronin's "Freedom City" world but other than that you'll have to get the supplement if you want to know more. Since I don't know enough about Freedom City yet to compare or contrast those two settings, all I can say so far is that I suspect Freedom City will have more of a four-color "mighty Marvel" feel to it than Empire City, which comes across a little more stoic, like most mainstream DC Comics.
Ultimately, they're both marvelous games (no pun intended) and I'm just grateful that, thanks to the d20 System, they're both more or less compatible -- and that means I can use products for both in the same campaign!
It's a really fun read.
