I'll preface everything I'm going to say with first mentioning that I haven't played Resistance online.
The ability to play 40 player online is not, in and of itself, that impressive. It has been done before many times on PC. The fact that it is the first console game to have that number of players is impressive however, and kind of cool no less. It bodes well for the future of online multiplayer gaming with consoles.
The nature of the co-op in Gears has also been done before in a few titles as well, but to my knowledge it's never been done so easily and seemlessly with the ability to have friends come and go while playing through the entire single player campaign. It's an impressive feature as well, and again, one that bodes well for the future of online multiplayer gaming with consoles.
I have played a ton of online games, of all types, on the PC and Xbox/360 for years. And my personal feeling is that I would rather have a tightly knit game of 4 vs. 4 in a game like Gears than a 20 vs. 20 in Resistance. Smaller groups make for better team play, better communication, easier ways to establish tactics, and more specialization. In a 20 vs. 20 game, it's far harder to maintain order, work together, and communicate, in my experieince.
Back in the days when I use to play Quake, and games were 8 or 16 player, I kept thinking how cool it would be once we got to 32 or 64 players, or more. But when those things came to pass, the games just got a lot more chaotic for the most part, and it took something away from the game, for me personlly. I do understant that plenty of people like those large scale games.
As a matter of personal preference, I'd much rather play a game of Gears on a peer to peer network (although I admit, servers would be better), than a game with much larger numbers of players where communciation is already a problem due to the nature of the system/network, on dedicated servers, any time.