They are more or less equal, the difference lies in personal skills and preferences.
op is a great tank healer as is merc, but that's it. aoe healing they suffer at
numbers - i have a 50 scoundrel sawbones with 2pc champion (+15% to kolto cloud) and +450 healing, and a fresh 50 sage seer with no gear and +372 healing
kolto cloud - ticks for around 400 every 2-3 seconds, crits for under 1k, lasts 15 seconds. 4 target limit, costs 30 out of 90 energy putting the scoundrel 10 energy above lowerin their energy regen rate ie. can cast nothing else for a second or two
salvation - initial heal for 650, ticks for 300 every second for 10 seconds. unlimited targets. costs 60 force out of 600 after a rejuve.
do the math
on a single target a scoundrel/op will out heal anyone as they can roll hots and infinetly do underworld medicine > emergency medpack. but mor ethan one person taking damage that requires more than keeping a stack of slow release on them to keep htem alive is where a scoundrel/op starts to suffer
sage/sorc? just drop rejuve then salvation and chill while your force regens. herpa-derp especially republic side wehre melee dps outnumber ranged dps