My pre-boxing main was a priest and I 5 box with a resto Shaman. My vote is Resto shaman and Shadow priest. Reason being the Earth Shield is godly, especially stacked with the Feral druid's self heal. Also for clearing trash the reactive heal/vamp should keep your tank up just fine and in my experience a resto shaman can do more dps than a holy priest (YMMV). For nomal instances my resto always DPS'd on trash to clear it faster. In other thoughts the high armor of the shaman is nice when they invariably take a hit but then again the priest has an aggro dump which will be nice since you're not running a pally for salvation. ToW, at least pre-WotLK will really help you crank out the DPS on the warlocks before you get good gear, so that's an argument for going elemental and holy. Chain healing is more efficient than the priest group healing... The VT return on mana + the mana spring should help quite a bit with staying power/down time as well. So, really afterall I don't think there's a clearcut answer here, you'll be fine either way and in the end Wrath will change everything.

On the lock thing, I think mixing spec's will add a level of complexity which won't return enough results to make it worth it. Keeping them same spec means they cast the same spell rotations and really simplifies things. Unless you set it up very well you're likely to have a higher DPS out of same spec then 2 different specs. Also note that with the focused coordination that we have you'll tend to kill things before your DOTs really have time to do reach full effectiveness so Destro is my preferred spec with a lock in the group.