As far as overall design and build-quality there is little or no difference. The rest depends on the pattern. Northfield often uses premium handle materials (i.e. stag, mammoth, etc) whereas Tidioute usually comes in bone, wood, etc. Also Northfield sometimes gives you grooved or slanted bolsters, pinched bolsters, long pulls, etc. Most (if not all) Tidioute comes with plain bolsters and nail nicks.
Again, what you get between the two depends on the pattern.