Honestly, it doesn't matter (don't start) - the young Classic Super Sonic is going to grow into that same, older powerful force that is Modern Super Sonic. That alone should be enough for a great match. What I'd really want to see is Classic Sonic learning from his future battles - i.e. Modern Sonic vs. Shadow, learning the tips and tricks from there. After all, he's seen doing the Air Dash in the ending - that's the sign of progress to Sonic's newfound abilities in Adventure/Sonic 4.
If I post on the UMvC3 board, note that I don't actually have intentions of getting the game. ~TouyaShiro (current name of FieraDevilWings)