Instance grinding with Rest is pretty competitive with Questing in Outlands for 60-70. If you know the TBC quests super well and you don't run into PvP issues, you can probably eek out more XP/hour questing. Blood Furnace is a spectacular instance to grind in, as previously mentioned. I'd personally recommend:

  • doing the easy Hellfire quest chains,
  • spending some time in Ramparts/Blood Furnace to knock out the juicy Dungeon Quests associated with them'
  • continue questing in Zangarmarsh (which I hate) or run Blood Furnace until you hit your mid-60s
  • quest in Nagrand, the mother of all kill-quest zones,
  • try another quest zone or start farming Mana Tombs, if your team can take it (I like mana tombs for its low-HP caster mobs and relative ease... it's great XP/hour, like Blood Furnace)
  • stay in Outlands to quest as long as possible to avoid PvP, or jump to Northrend at 68 to start questing

Northrend instances may be tough at first for a team of 68-70s with non uber gear. (This depends on your group makeup, however)

I really, really, really liked instance runs for levelling in WotLK, but I play on a PvP server and don't like being interrupted when I'm trying to level. I probably did 50/50 questing/instances from 70-75, and leveled almost exclusively in instances from L75-80. (I am questing with my team at L80 to rake in cash for mounts, now). Upside of instance running is that you can pick and choose which instances you want to run and accumulate a pretty good set of blue dungeon drops for your team. You have the flexibility to farm only the bosses you might need-- I ran Halls of Stone 30-40 times and just killed the first boss or two until my team of hunters all had the gear drops they needed, can't do that in a normal group so easily...