I've done both... luckily both those classes can dps/heal... anyways Merc - less burst dps more channeled abilities that can be interrupted, has amazing def cds to mitigate dmg... Op - huge burst lots of stun dps, stealth, vanish, dot remover... tons of fun to play pvp... Neither of them can be put to WoW classes (if you played) but if I had to, it'd be Op is rogueish and Merc is Balanced Druid ish (DPS ONLY)... both can off heal and without being spec'd its not very effective... The big difference between the two classes, imo would be the end game opportunity... Merc can rDPS and (tank) heal Op can mDPS and (raid) heal (parentheses since I use those terms loosely) if in the end you want to dps, most raiding groups would pick the Merc over the Op due to the fact there is less rDPS in the game than melee, and sustained dps is greater than burst in a 10 min fight