I used Game Maker for years.
more intuitive, even for beginners. Elegant events sheet. Construct have a nicer level editor, since you can position, rotate and scale instances as you want. In Game Maker, you can't.
Faster executables files, since Construct compiles exes, Game Maker interpret at runtime. Faster 2D engine.
Construct uses DirectX 9.0c, Game Maker uses DirectX 8.1. Construct have shaders, and you can even make them. Construct expansion is virtually infinite, since you can make as many plugins as you want.
You can make games with Construct. In Game Maker, you can't.
Then again, it's Open Source. It's a great game making tool. It's free. It's powerful. It's so good and it isn't even on 1.0.
Okay, no online. Someone is making a plugin for it. Remember: Construct is free, Game Maker costs 15-16$ and it's old, slow and dirty.
p.s.: Game Maker Community existed for 6-7 years. This community, since 2007 if i'm correct, so 3 years.