(danger will robinson!) okay which is better?
and
can they be rechambered?
LOL! That's a loaded question, if you'll excuse the pun. I find my Mini 14 to be slightly more accurate than my Mini 30 but that could be due to the quality of the ammo. Also Ruger recommends that you Do Not fire steel cased ammo through the Mini 30 which kinda cancels out the cheap AK ammo advantage.
IMHO, all things considered I lean slightly towards the Mini 14. Does almost everything the AR15 does (sorry no bayonet lug) and right now ammo is readily available. Plus if you look around you can still find 30 round mags that fit both the Mini 14 and the AR15. If you want to go with 30-30 ballistics you won't regret getting a Mini 30. I like them both.
and can they be rechambered?
More trouble and expense than it's worth. What caliber are you wanting?