From what I noticed so far - magden is great at supporting, giving your group buffs, slowing enemies, healing etc. - doing all any self respectable support would do. However magden's DPS is lowest of all classes, I even watched someone (was it Alcast? I think so at least) test DPS of all mag classes - warden scored lowest, and could not even get to 30k (it was getting to 26k) whereas even magblade easily went over 30k (and keep in mind magblade was considered weakest dps).
Stamblade however offers good dps and great burst (if you can time your skills well, you can get your enemy 100-5/10 (low enough to be killed with the next blow) within one second. Prep Subterranean Assault, cast Cutting Dive and use your charge (if you have 2h). With good timing, Assault will hit just as you charge and your Dive hits, leaving your opponent wrecked. Pop your bear at that moment and it's K.O.
That is PvP of course. In PvE stamden still offers respectable DPS (quite better than magden), and you can still spread some buffs, however group utility goes down while DPS goes up. So basically it's a tradeoff - utility for dps. Myself however - I prefer stamden, not just for numbers but because as magden I feel gimped. Templar can also heal from behind and do more damage while also offering team support via buffs, regening resources etc. - hence I'd rather pick stam, I feel as if I'm doing more.