i think there are 2 sizes on the rears, but don't quote me on that one

the pair may also mean the 2 seperate parts as they slip in from either side rather than get pushed through the middle like the oe bushes do

price is also a big factor, powerflex is what i'd go for but deflex also do them for about a 10th of the price

i've got a whole rear end sitting in the garage from deflex if you want