I'd say Sonic Colors and Generations are on pretty equal footing, they both have strong core games with a lot of extra challenge modes that can be hit or miss and ho-hum bosses. I do like that Sonic Colors is more consistent than Sonic Generations and has really cool power ups, but I also appreciate the nostalgia, visuals, and classic Sonic in Generations. It's good to see Sonic on the up and up though.
I agree about the bosses too, but considering they had so many classic Sonic games to pull from, I really wanted to see Robotnik show up with the giant ball swinging from his hovercraft, or the drill car.
Sonic 3 definately deserved representation; Hyrdocity or Ice Cap Zones would have been great, and Sonic and Knuckles could have had Lava Reef for a fire and ice motif...but alas.
As for the level listing, here's the Wikihttp://en.wikipedia.org/wiki/Sonic_Generations#Levels
The 3DS version will also apparently have special stages, something I felt the game was missing.