Anyway here's my take on S&M at the Olympic Games, and surprisingly it's rather more positive. Earlier release date? Doesn't make the slightest bit of difference.
S&M is a minigame compilation, a pretty solid one at that from what I've read from an IGN hands-on I believe it was, but it's riding on 3 things to provide a boost to the sales.
1. Appeal to casual gamers
2. Olympic games (even if it is a bit early, it might sell more at a later time period)
3. Sonic and Mario appearing for the first time ever together in a videogame
The reason why an earlier release date is necessary as such is because if it released later than Brawl it will lose one of it's major selling points and SEGA needs games with major selling points to keep the Sonic cashcow afloat. Brawl will be great, garunteed, it will get awesome reviews and sell millions to everyone who loved Melee and maybe a new crowd too. Hell it'll probably even shift a few thousand Wiis. It doesn't NEED the appeal of "First Mario with Sonic game ever!" because it'll sell incredibly well regardless and as long as Sonic is in the roster, the fans will be happy. Your beef isn't with the fact that Sonic is appearing in another game first, it's with the fact that you're worried that it'll upset his chances of being in Brawl and to that I say "Rediculous! Never!". As long as Sonic is in Brawl it'll be the first time that Sonic ever fights Mario as such and all the other Nintendo characters and that'll still be a major selling point to Sonic fans (who long for decent games with Sonic in ^^).
I hope you see why an early release date makes sense for S&M, if it was the other way around Brawl's sales most likely won't see an increase whatsoever (Sonic will most likely not be revealed before Brawl's release is my betting) and those fans that buy it will have 1 less reason to buy S&M, making that game suffer in sales.
I can honestly say this is the best for both games. Sonic will be in Brawl, regardless.