IMO if you are focused on HTML5 games and you are already familiar with Construct 2 then you may stick to C2. C2 has very nice performance for HTML5 games, it's very user friendly and you can do whatever you want in a fast and simple way.
However if you are focused on Android games, then for now C2 is not that good as Game Maker especially regarding performance. C2 is meant to be used with Intel XDK with Crosswalk. Crosswalk works great with devices like SGS6 (devices which are at the moment the strongest ones available), but older devices have problems running dynamic Crosswalk apps smoothly.
So to wrap up IMO:
Construct 2 - a must have for making HTML5 games and very simple mobile games (I believe this limitation should be gone within not more than a year)
GameMaker - a must have if you are focused on mobile games and/or if you are a programmer. But to be honest, it's a sin to not buy it while you have a chance to order it for $12 instead of $450 anyway.