When looking at buying new disc routers for disc brakes, what's the difference between say 200mm ones versus 160mm (other than 40mm)? My buddy is buying a new set of wheels to put 'road' type tires on his mountain bike, with a more appropriate cassette for road riding.

I found some good wheels but they don't come with the routers. So I'm looking at different ones, and MEC has 203mm for $35 each, but 160mm are only $15.

Any insight?