Well having used (and now using only C2 :) ) both i can tell some pros and cons:
(Comparing with HTML5 version of GM when pertinent)
Construct wins on:
Much better drag and drop event system
Much better looking and well organized interface , more usability, less clicks etc.
Plugins system is imo more powerfull with plugins and behaviors
Better game performance
Less time between releases
Layout editor is more flexible and easier to use, even if it not support tile maps yet.
Better out-of-the box experience with super useful and more powerfull plugins and behaviors like platform , physics etc.
Overall: Much more productivity
(Obs some of these are personal opinion other are from sources like blog posts)
Construct "fails": (compared to GM 8.1 based on feature list on site)
No internal image editor (hopefully to come soon)
No tile map support on scene editor
No built-in particle effect and editor
No support for 3D (yet to come on HTML5 version of GM)
No support for customizing load screen
Bad font and text capabilities (Hopefully to change soon)
Less/no support for data structures like lists, queues, dictionaries, stacks etc
No built-in path movement and timelines
No built-in AI-Pathfinding
No support for external import of JS code (Uncertain of this one, please correct me if it does)
No support for scripting. (Would be awesome but the event system served me more than well till now).
No built-in support for version control, team work.
Only exports to HTML5 currently
That's it. There're pretty nice features missing but it's not fair since Game Maker has been around for ages past Construct2. But i'm sure C2 will catch up with most of these features eventually.