Yeah, neither is perfect but both are pretty amazing when faced with the "wander around looking for the thingee with the sparklies" alternative. Carbonite does take over more stuff then just "quest helping" and it's debatable whether or not those extra things are really a plus or not. On balance, for me, though, QH started sending me on wild goose chases so Carbonite worked better. Functionally, both QH and Carbonite do the whole quest planning / arrow pointing / kill this-mob-for-this-quest info thing that is the big helping point.
On the flip side, since I am totally in last place on the race from 70 to 80, for fast leveling maybe you should do the OPPOSITE of what I do...