@sdyates
A few AGA only games that spring to mind are Bloodnet, RoadKill, Banshee, James Pond 3, Guardian and Slamtitlt pinball. Also most the FPS games that appeared were AGA only. I'm sure you can find quite a few more if you look.
Most AGA versions simply featured more colours than their ECS counterparts. Some of the pinball games featured a high res view so you could see more of the table and super Skidmarks also had a high res mode.
As for wether it's worth getting an A1200, then that depends on how cheap you can get one. Also if your OCS/ECS amiga doesn't have a hardrive it is a lot easier to add one to an A1200.
If you add a hardrive you can use WHDload and JST to install and run your games from hard drive. You can probably find installers for all of your existing games and most legal download sites have HD installed availble along side ADF versions.
Also with an hard drive you can easily write the legal ADF games back to floppy too.
If you like the idea of running games from hard drive you will need some extra fastram too.
