They are both made in the same factory, usually made of the same materials and off of the same templates, so there really isnt usually much difference except mark todd rugs sometimes have little extras that JHL dont.
I'd say it's a bit like Masta doing Zing rugs and W/beeta doing Saxon - you're paying more for the well-known/prestige brand. I own Zing rugs and they are going strong after several years. If you're on a budget, save your pennies, but if you feel like treating yourself, splash out. Nowt wrong with JHL rugs overall though, they're very good.
I work in a tack shop and i think JHL are way better in fit, comfort and long lasting than Mark todd any day. the MT fly rugs are not proving hardy this year!!!