I didn't know about these Sonic games, but as a Doom fan, I have to say that the use of the Doom engine for Sonic makes good sense. The way it feels to move around in "3D" space in Doom is one of the best parts of the game, IMO; there's a weight and sense of momentum to the player that I found lacking in full 3D games like Quake. I could see this working very well in a platforming/kart racing context.