This game was alright. I surprisingly had a good time with it, mostly.
Felt the same way about your feelings toward each of the Sonics, and got the same vibe that they were trying to shut me up about my bitching of the games of late by forcing me to use modern day Sonic along side of the guy I liked better, but thanks to Modern's god awful controls, finicky physics, and inability to shut up for five seconds, I found myself appreciating the classic version more. I actually thought the first cutscene with both of them amusingly appropriate; Modern Sonic can't stop flapping his gums long enough to get to business, while Classic is already gone.
Still, I had my share of fun with both shades of hedgehog, even if also my share of frustration. This one gets a surprised thumb up from me.