me in fact a gamemaker user, and i still use it, but for Construct 2 their some aspects why i like it over gamemaker in some cases, here the reasons:
1- the behavior system: like the platform which can be easily add and change, useful for prototype and fast forward developing.
2- the webgl shaders effects: something may be hard in some other tools to add without hassle of coding.
3- physics without coding: yes, construct 2 support physics almost easy for non-programers, something almost ignored or limited in other tools.
4- being gamemaker user, can benefit you to use c2: construct 2 allow you to read the sprites/tilesets and levels from gamemaker, this just a great feature for all gamemaker users, we barely see this happen in other tools like the old days.
of course, gamemaker also have an advantage points over C2, like native mobile support, tilesets, and gml, sometime a complex function might be easy in gamemaker using GML, while it's look messy and became nightmare in c2.