Well all 3 are horst link 4 bar DH frames.
the big hit is the old skool standard 4bar variant, Demo a newer lower COG and stiffer version while the YT seems to have an almost virtual link thrown in too for good measure.
They'd all get the job done.
the fork and shock on the big hit are not great and parts are all cheap (but alright for the cash)
The YT is an absolute bargain, throw the frame straight in the bin and it's still an unbelievable deal for parts alone.
geometry wise the Demo wins as a WC DH bike (slacker/longer and lower)
even if you decided to race world cups you shouldn't need to spend another note on either the Demo or the YT after you leave the shop.
But if you brake the frame the Demo or the big hit are the ones to have.
I'd go with the Demo but that's me, not you.