I'd say probably Crucible, especially if you can run with a GAF crew to make the matches go faster. Plus taking bounties.
Also, just do the story missions to advance the story enough to complete any good bounties for the day. Like if there are a couple 5,000 XP ones on Venus then just do the mandatory story missions to unlock them. Or you could just avoid all story and bring your Level 2 into Crucible ASAP, haha.
I saved a lot of the story missions for once I hit 20 so then after I transfer my gear, I went back to the story and did the Level 30 missions for 7,500 XP each time, which makes it much faster to level the alt sub-class. Doing the story missions first wastes all that XP if you're a Level 10 doing the normal Level 10 version since you can't repeat the story at a higher level for more XP -- so stupid.