I've just discovered Swtor a couple months ago and have loved it!! i have a lvl 22 infiltration shadow and a lvl 25 heals/damage sage, so take this with a grain of salt.
I have to admit that I like the shadow the most for pvp. They do a massive amount of damage and have 3 stuns that make then something to deal with in pvp. I've especially enjoyed cloaking and sneaking behind pvp lines to get rid of the healers then vanishing with half health. Heal and repeat.... FUN! PVE is nice because you normally get the first strike which if you're a nerd like me and 4 levels below your mapped area is very handy. There is a little bit less thinking involved than the sage. Just go and beat on stuff until it dies.
The Sage is a bit more rounded but tricky because you have to rely on your shields, stuns, damage over time, and healing to survive. PVE is tough because of the need to manage what your companion attacks during the battle. SWTOR has one thing on WOW in that the enemy group will target the healer. If independent just task your companion to attack the npc and they normally avert their attention. Easy in normal encounters and when you're a level above where you are, but in heroic it's tricky. If you play your cards right and manage your companion well you can clean out a same level Heroic 2 and even a Heroic 4. Takes time but it's doable.
PVP is another story. 1v1 YOU ARE IN BIG TROUBLE! Especially if you are up against a tank type that knows their specializations and how to play their cards. I've gone up against some tanks who destroyed me and walked away with 4/5 or their health. NERD RAGE and EPIC FAIL )-:
If a Sage is left to it's own in a group of at lest 3 your team is next to unkillable.
Some tips that I learned when I played as a sage: Most people target the opponent with the lower health, so keep your health next to full and most of your troubles with enemy pvpers tend to go away (with the exception of enemy bounty hunters who don't like you for some unknown reason). Then your job is to just maintain your team as best you can. This is really hard if you click on your allies toons. You will click on an enemy almost every time. ("RAGE") I've found it's alot easier to just pay attention to team box on your left and select them from there. All the stats are right there. I've found that Redemption is more "Force" cost effective than benevolence especially if you use the specialization for shorter casting time. I aimed for shielding and healing and selected the specialty for the 20% more shield absorption. Shielding is a high cost so don't use it if it's a long drawn out fight, but if your teammate is running low on health shield him then heal. The shield gives your ample time to recover the toons health and you're on your way.
I've found that there are alot of healers out there, but a good healer is hard to come by. If you are not a healer please mark the guy and keep enemies off his back. If you play pvp and your healer is left to his own devices you are a force to be reckoned with.
Drawing battle lines are great for healers, but I'm rambling now.
Thanks for Reading and good luck on your questing!
- North