Hi Cilidar,
Hopefully I can help you decide which one is right for you.
2nd Edition has a lot of powers, maneuvers, and optional rules as well as a couple of additional statistics (namely strength and tonnage). But no kingdom rules. 2nd edition also used facing rules. I would say it is slighty more complex than 3rd edition but not by much.
3rd edition has kingdom special rules, condensed powers and maneuvers, no facing rules, and is part of the Toy Battle System. Many of the unique powers and maneuvers that aren't in the GMR 3rd edition book will most likely appear in a different TBS game that suits it better.
In the long run I think GMR 3rd edition and the other Toy Battle System games will be a better group of games than GMR 2nd edition.
Currently in the works is Power Warriors (Ultraman, Dragonball Z, Street Fighter style of combat) due out in November and Mega Bots due out early 2010. There are also plans for swarm and super hero TBS games planned for 2010. All TBS games are stand alone games that can also be used as expansions for any other TBS game.
I should have a preview of the Power Warriors cover up on the website sometime in the next few days followed by a preview of one of the different styles of fighting a warrior can be trained in.
Hope this helps.