I'm playing with both of these classes trying to choose the one with more high end pvp potential. Here is how i have them broken down:
Merc(Pyro):
Incredible AOE DMG, above average single target dmg.
Decent survivability through mobile dmg, knockback, shielding, heavy armor and healing.
poor utility, electro dart, jet pack.
Notable weakness
No interrupt.
Notable strength
powerful instacast ranged dmg.
Great hybrid healing.
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Operative(conceal)
Incredible single target melee dmg, decent single target ranged dmg, poor aoe dmg.
Low survivability, virtually none if vanish is down.
Good utility threough flashbang, debilitate, disrupt, jarring strike.
Notable weakness
big target when out of stealth, lack of knockback or mobility makes teamwork a challenge.
Notable strength
Energy and health HoT with stim boost makes this class the strongest energy management.
Well rounded toolset.
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Would like to hear some other opinions on this comparison before i push one to 50.