better build accuracy (they pay more attention to inlet manifold sizes, valve sizes etc etc...) just overall quality is better, material is better built etc...
speed wise there is little difference but the lifans have been more reliable than the ducars generally. sometimes you will get a freak ducar which will really run hard and really well but more often than not, you will spend more time fixing them than you will in a lifan. remember Ducars are made cheaper than your lifans or GPX engines which would generically suggest that the lifan and gpx are just plain and simply better build quality and better materials which is true.
hope this clears some things up.
Cheers, Tony.