It really depends on your play style.
-Bards have more survival options and more tactics in PVP like ode ruby and resonance point and some other awesome stuff..Also bards have the option to have some heals (Nocturne) so you won't have to use your martyr blessing heal thing and waste mirra and they also have energy instead of mana and not to mention they almost have no CD s
.They even have a skill to reset all CD s (the highest CD skill I've seen for bard is 5 min CD and that's invigoration rubies if you have both rubies and encore which resets CD of almost all skills has less than a 5 min CD and can decrease depending on quality of tears of dragon you use) and moreover, Bards' skills can be done 24/7 like chords and AOE s that you can do all time and any time and solo (in PVP) and when you use muse skill hehe so simply the only thing that binds you is energy and bards have tons of energy rubies ..Additionally bard doesn't offer only DPS but also support like Rhapsody and Nocturne and Ballade which actually makes them unique in PVP since they can be near invulnerable against CC using ballade and acoustic barrier rubies even with low will power.Also bards wear leather unlike mages who wear cloth.
-Mage,well I tried mage reincarnation before and couldn't bear playing it past level 13-15 so as far as I noticed it has cast times unlike most of bard's skills yet mage can offer a higher DPS potential while they're squishier than bards specially if there stony barriers or elemental barriers are broken and as far as I noticed on DPS meter etc Mages and warrior DPS (specially warriors with frenzy buff (_ _) and crushing blow) get highest DPS on meter most of time but on the other hand mages don't seem to support as much as bards..The only support I noticed they could offer is the ice grave thing that CC mobs and bards have a kinda equivalent skill from a ruby called lullaby.I'm not so much into mage after all so a mage can tell you more what a mage is capable of (pssh they sometimes 1 shot kill people in PVP :O)
Anyway it's up to you and your playing style as I mentioned earlier,I for one am not fond of annoying cast times and non spammable skills and CD s so I find bard more fun but surely many people see mage more fun