For obvious reasons, I won't list common features.
Pros:
- more freedom (you can do things that you can't do in GM)
- free (you said it yourself)
- powerful (it can run stuff smoothly where GM would choke, turn blue and purple and then keel over)
- easy to learn and get started
- very easy to prototype games
- one of devs is a kangaroo lover
Cons:
- still in development (not finished, though it can be used to make and release games with, but it is nearing the release) - expect stability issues, bugs and stuff
- still difficult to develop a major stable project
- still has some annoying limitations (that are being addressed in Construct 2)
- wonky scripting support
- the other dev is not a kangaroo lover
All in all, in my opinion Construct is an excellent introduction into game development, however for more serious projects I'd wait until the final release (soon!) or even Construct 2 (which won't be any soon).
If you ask me for opinion on switching to Construct, well, it certainly wouldn't hurt to give it a whirl to prototype a game or couple. You might be surprised, who knows? However, be aware that while it may be easy to get into Construct, it is hard to master it as with everything else. As of now, I've yet to see a complete major game made in Construct.
That may change with the final release version, however, as everyone is pretty much putting their major projects on hold until then.