just pure speculation here, but an xtatic set is better for its accessibility. a +whatever pvp magic set for your level is better for survival and damage
physical defense is fine and all to have, it's only if you can actually make use of it by being a class that works around having reasonable amount of stamina
but since we're talking about mages and co. here, having the usual amount of phys def from w/e class armor set you get your hands on (since mages aren't naturally temps with over 9000 hp and stamina to begin with) doesn't stop you from getting cracked every so often. then again you can just stack stamina on a coli set if you're funded enough to do it, for the rest who can't, it'll take longer; but at least it's faster than getting an entire pvp set up to +3 and above. it's a lot more tedious to get and a lot more infuriating to enchant if you keep failing
otherwise the additional 500+ hp from each piece is not all that different from the lowered (what, around 300 less or something) damage from the usual xtatic/class armor set except you don't get cracked as often. could be different with a coli set though
if you still need an example on how you can benefit from a pvp magic set...
-stares at jerry-