You get more XP if you team, because you can take on Heroic quests, Flashpoints (TF equivalent), and eventually Operations (Trial/Raid equiv), all of which have higher concentrations of high level enemies that give significantly more XP. You level *faster* if you team because you benefit from their sidequests that you don't get (class quests, essentially) and a team tends to mow through the content a lot easier than a single player. Also, you can take on random Champion rank enemies (think EBs) that spawn in the wild without dying, which is additional XP. Most of the given quests aren't instanced, aside from storyline quests, so for the most part, you don't get larger spawns if you team.
Also, for me on Begeren:
Priss: Jedi Knight/Sentinel (Dual sabers DPS)
Ne-Nei: Sith Inquisitor/Sorceror (Ranged Lightning DPS)
Keija: Bounty Hunter/Powertech (Ranged DPS/Tank)
---
"Oh, silver blade, forged in the depths of the beyond. Heed my summons and purge those who stand in my way. Lay
waste."
Also, for me on Begeren:
Priss: Jedi Knight/Sentinel (Dual sabers DPS)
Ne-Nei: Sith Inquisitor/Sorceror (Ranged Lightning DPS)
Keija: Bounty Hunter/Powertech (Ranged DPS/Tank)
---
"Oh, silver blade, forged in the depths of the beyond. Heed my summons and purge those who stand in my way. Lay
waste."