Sonic Worlds is a fantastic engine, it's highly modifiable, and easy to use. you'll need MMF2 before you can use it however.
Personally I found GameMaker to have one of the WORST user interfaces I've ever seen, It's cluttered and difficult. In MMF2 you can keep each object not used in gameplay off the frame, for simple navigation, also you have a handy dandy folder system to keep things streamlined.
Sonic Worlds also lets you use Knuckles and Tails, and once you're familiar with the events system, programming is a breeze.
http://www.sonicfangameshq.com/forum/showthread.php?t=6221