RE: Mini-14 in 6.8SPC or Mini-30
I always use the analogy of gasoline in cars to cartriges or quality of ammo fired in a rifle.
For instance, it makes no sense to buy a $2000 precision rifle and then fire wolf ammo through it- its like buying a corvette and running it on low-grade gasoline.
The converse can be said about the Mini-whatever. It makes no sense buying a craptastic rifle and having it chambered in a cartridge with better ballistics and which is much more expensive to shoot. Think of it as buying a Geo Metro (albeit grossly overpriced Geo Metro) and running nothing but premium gasoline in it.
The Mini-30 is the way to go, if only because the SKS isn't available to you.