hi
now this ios my area of knologe, since i own a mountain bike shop as well
now i would say that if you have different tyres for the front and rear of a bike they are most probilly cheeper tyres
(sub 15 ea) when i would say the more expencive the tyre the better it is, but i dont expect you to go out and cut up a £50 tyre(an yes that is a common price for some).
now as for the tyres u have, one of your should have a tread patteren that goes across the tyre(rear) and other should go with the tyre(front) now i would use the rear as this should give you better grip due to the tread patteren.
if you want any more advice just ask me