Man, there was just like a whole entire thread about this very subject. Anyways, here goes again:
The Greatest Hits version is NOT debugged, however it is significantly more user-friendly. There seem to be 3 main bugs in Greatest hits (also known as version 1.2): sound dropouts during long races like enduros and the longer race series, #2: cars merging, and #3: the Garage Thief. You'll really only have to worry about #3, which can wipe out your entire garage. The car merging thing rarely happens, and it's funny when it does. The sound dropouts are annoying but don't affect your visual gameplay at all.
To avoid the Garage Thief, don't use the Machine Tests. If you MUST use the tests, don't save your game afterwards (when it asks you to save). Instead, check the game records to make sure they're not funny looking, and check the first car at the top of your garage to make sure it's got the power & weight that it should. Then you can save the game in the main menu.
But if you never use the machine tests, you'll never have to worry about the thief and you can save your game whenever you want. You won't have to save it twice either.
