If you die, you screwed up. There's really no human error to worry about than your own.
You can get 200 exp a kill for VT mobs soloing (if your pet doesn't die) or 60-70 exp a kill for VT mobs in a party. You can probably chain the VT mobs to get up to 105 exp each, but the majority of parties are mediocre at best and run out of steam at the first chain. Plus, the WHMs and BLMs take longer to heal than a BST/WHM ever has to, and the BST has less downtime whether he catfights equal cons while he rests or not.
You can't really trust my numbers here, because they didn't totally add up when I checked the exp division charts. I'm going by what I saw, though.
Either way, soloing is just more favorable to me because I never have to deal with stupidity. (Would you believe one of my parties was slaughtered because a RDM/WHM didn't know Diaga was an AOE spell?)
If you wanna have the solo option though, you don't HAVE to be BST. I've seen successful WAR/WHM, PLD/WHM, and SMN/WHM solo past level 25, which is nothing to laugh at.