in the case of multiple mobs MRD wins handsdown... infact in this case i would say MRD is a requirement because it is close to impossible to keep reliable enmity on multiple mobs (3+) with gladiator (post 1.18)

the best example is probably mosshorn HNM, in which case it is ideal to have one gladiator tank mosshorn and a mrd tank his minions.. otherwise you will need a party full of gladiators.

on the subject of single targets- a good gladiator should never have a problem keeping hate, so in terms of damage mitigation a gladiator will always win

however, you could also say that if a MRD can keep more hate this will free up the archers and thams to really let loose, that is really micro-managing though.

to sum up, gladiator and mrd are both excellent tanks and anyone serious about tanking should rank both jobs and excell in all tanking situations.