Sonic Colors is one of my favourite games of all time. I picked it up when it hit $20 because I had grown tired of bad Sonic games (Secret Rings was ok, Black Knight was terrible).
The use of the color powers are much better in Sonic Colors, the soundtrack is much better. I would have been happy paying full price for it.
Lost World was fun. Different kind of gameplay. The controls for some of the color powers were confusing. Worth a purchase but I did not enjoy it nearly as much as I did Colors.