First I want to say i'm GM fanboy yet.
I love GM so much.
GML is simple but powerful for making 2D game. And GM's interface is better in my opinion.
But the runner is so slow and crashing sometime. Hm, graphic in gm is so bad. Though that i can make a "big" game with GM.
I'm newbie with Construct but I love this program too.
How it runs smoothly and very fast. So many build-in d&d functions are easy to use. And because i'm studying about python, it's more better. Construct is more easy than GM yet.
But something crash for me with Construct. When i try to make a little big game it's really horrible, some message's bug and interface was slow. And d&d is not good enough. So i will use GM with my main project. I fear that crashing of Construct at working. If Construct have something like GML, something will be changed, i can leave GM and give all of my money savings to Scirra.
In my opinion, GM is beating Construct yet. Maybe Construct is younger than GM and was beeing free working. I'm interested with future and going to use both.
Sorry for my English.