The platform behaviour in Construct is amazing. For platform games I would prefer Construct over Gamemaker.
Yes Gamemaker can play in a browser. But I believe that is a very simple activeX plugin. I think you can fairly easy create ActiveX plugins for any application so I see no reason why Construct would not be able to do the same (in a hopefully near future).
The main reason that I pick Construct over Gamemaker is that the Construct layout editor is so much better than the GameMaker room editor. Its things like layers and locking objects. In general editing levels in Construct seems so much easier than in gamemaker.
The event action system in Construct is fairly strong but when it comes to more complicated stuff i would so much prefer a scripting system like GML. (or even better python
The forum support for Construct is very good. The autors of Constructs seems to be very active on the forum and on releasing new versions of Construct. This is another top reason to pic Construct.
In gamemaker Objects are more flexible in the way that you have to create the behaviours your self. In Consturct there are a lot of build in behaviours... but they are limited and you can not easily extend Construct with new behaviours. I wish there was a way of creating new behaviours using python.
At the moment I actually prefer Gamemaker over Construct for most tasks. But Construct has a lot more potential in the future.