I feel the same way. I leveled as a medic, and experimented with the various DPS companions, but I always ended up going back to a tank.
The DPS companions would lose agro, and I'd end up having to waste globals putting kolto probe on myself, just for throwing in a backstab. With SCORPIO or Kaliyo, as long as I'm on the same target, I rarely pull agro.
Maybe it's my playstyle that works better with a tank though. I didn't really enjoy the whole aoe spam, rest, aoe spam, rest. I preferred to sneak up, sleep dart one, and single target them down. By the end of the pack, (most of the time) I'm at full health (companion too), and full energy by the time I reach the next pack. I'd rather do it this way and kill a bit slower, than have to rest a lot....I hate resting.