The commando/mercenary both have pretty engaging stories that I would consider much better than the sage. The closest thing I could compare it to is a chloromancer from Rift, otherwise you could think of it as a pretty much single target healer. The scoundrel story I did not read as I was focusing on trying the class before beta ended, but from what other testers have told me the Operative has the best story of all the classes. The scoundrel/operative healing will make use of the cast heals, hots, and earning points through those heals to use other heals which help you balance out so you do not run into energy problems and go oom very early. This class also makes uses of the cover mechanic which is interesting and will add a lot more strategy to the class (especially for holding a area). Along with this the Scoundrel/Operative gains the use of stealth out of combat as well as a in-combat stealth which I believe is a 3min cd.
Both of these classes make use of a energy/ammo for abilities which recharge faster when you have over 75% available, making these classes ideal for steady heals. If you are wondering however which healing class will be most looked for in raids, I would recommend the Scoundrel/Operative as they appear to be the most underplayed class in the beta's and are even rarer for the people to choose the healing route, but this class offers unique abilities that will make it essential for at least one in a raid. I hope this answers your question. GL with w/e class u choose!