Which is better - Sonic unleashed or Sonic colors

Depends on your choice.

At sonic unleashed WII/PS2 version is sonic colours surely better.
At sonic unleashed XBOX360/PS3 version it depends on what you like, the most will like sonic colours more where it's more based on speed. At sonic unleashed you have a lot of werehog stages which are slowly and in my opinion very good, but doesn't fit in a sonic game.

This is just an opinion of me, maybe you think different about it so to find that out you can watch some gameplay movies on youtube. ;)