 The ending depends on how many Chaos Emeralds you have.  If you dont have  all 7 Emeralds, Sonic will jump off the "Sonic F" plane and pose.  If all  were collected, Sonic will transform into Super Sonic and fly and pose while  soaring.  If you choose "Tails Alone", Tails will fall, Sonic will catch  him and pose if you collect all Emeralds or not.  After the credits, if you  collect the Emeralds or not, a sign will say, as always, "Sonic the  Hedgehog 2"