If your not a programmer Unity, AGS and Wintermute are going to be hard work
Unity tends to be a money pit, I know a few people have ended buying plugins to get the job done
There's a game engine called AGE which works cross platform, I've had its demo game working on a Kindle Fire http://age.seccia.com/
Adventure Maker and Visionaire are nice newbie friendly engines, Visionaire has the edge on features and has proven successful for commercial games
SLUDGE is one of the first, still maintained and works on Mac and Linux as well as Windows, maybe a bit primitive but it is geared towards open source codec and the source is open source too
If I was just starting out I'd find one I can do a basic level without having to read an entire manual, look at the games people have done, and see how friendly and supportive the forum is, on that front, you'll find Wintermute has tumble weed blowing through it, AGS has to many EXperts and can be aggressive towards new people, a topic like this would be blasphemy over there!
Like Lee said read the licenses, its a nightmare doing cross platform, personally just do something, try and make a small game, get feedback and learn by your mistakes, going all commercial is a shitty minefield if you just dive in expecting the biggest best game ever you will fail :p