nothing and everything...
At a short distance, cosmetically, there is little difference. They look pretty much the same and function pretty much the same.
Where they differ is internally and in strength. The trigger on the Encore is generally not as good as the Contender without a little work. The Contender(Old style) is adjustable for creep and overtravel. The G2 is not, nor is the Encore.
Strength is the biggie. The contender is a weaker frame and while it can handle powerful cartridges ie 45-70, 444 marlin, .375 JDJ. That's about where it maxxes out. The Encore can handle any cartridge available. If Magnums are your thing, then the Encore is your gun.