In a nutshell,
The Contender will handle a very wide variety of chamberings both rimfire and center fire and there are many, many sources for barrels. As a rule of thumb the chamberings for the Contender are limited to lower pressure cartridges around +- 45K psi.
The Encore, with its stronger frame construction will handle all the cartridges of the Contender plus all of the high pressure rounds to boot. From the factory the Encore won't shoot rimfires but a conversion kit is available to enable the Encore to shoot rimfires. Like the Contender there are many, many sources for barrels
I haven't heard a whole lot about the G-2 Contender. Designed like a 'mini Encore' I imagine it will handle a little more pressure than the old style but I don't know if it will handle as much as the Encore.
For obvious safety reasons Encore bbls will not interchange with Contender frames.
I like shooting diffrent calibers and bbls are cheaper and easier to aquire than complete guns. The old style Contenders and 14" bbls. are my favorite. True, if you release the hammer you have to 're-cock' it by opening and closing the action, this could prove to be frustrating in an intense hunting situation but so far I personally have not been inconvenienced by it. I like the small profile of the 'old style' Contender and the adjustable trigger. Being a paper puncher I don't require big boomer barrels, just accurate ones. For the little bit of deer hunting I do my barrels chambered for 30-30 or 44 mag are more than adequate for the little blacktails in my area.
Like any gun, they are only as safe as the person holding them. A word of warning tho if you are considering either an Encore or Contender...
You will either like them or not, however, if you like them they will create an addiction for barrels that will sometimes over power all reason and of course with new bbls come new dies, brass, bullets, powders, etc. etc. They are plain and simple fun.
For more info check out
http://www.bellmtcs.com/
http://specialtypistols.infopop.cc/eve/ubb.x