The Encore is a stronger frame, capable of handling cartridge pressures that would tear apart a Contender or G2. The G2 is the new version of the Contender, for lack of a better description. The Encore is centerfire only, while the G2, like the Contender, can be switched from centerfire to rimfire.
If you like to shoot a lot or want to practice inexpensively, then the G2 or Contender may be better for you as it can shoot the .22 LR without special barrels or adaptors.
The Contender and G2 can handle many cartridges up to about .45/70, including .223 and .204 Ruger for varmint hunting. For higher pressure cartridges (.243, .22-250, etc.) then you would have to go to the Encore. Another advantage of the G2 is that it will take most Contender barrels. The Encore only takes Encore barrels.