heya Brigh,
Prolly a question for others that are more active like cern,eko, xan etc. At a guess though i'd say go to the starter zones at level 68 and do a few of the early quests, then take the rewards and go back into TBC until 70. I think you prolly could just stay in WOTLK but it would be frustrating i would imagine as many quests would be unavailable cos your not high enough level.
Remember with TBC the rewards were ultra silly and thats why everyone couldn't wait to go there, the WOTLK rewards are far less powerful - worth getting a few upgrades but then you start to see similar stuff for quest upon quest. Cant really comment if TBC exp is worse than WOTLK, don't think it is but maybe see whilst you're there.
Leveling rate felt kinda slow to me, but having said that i wasn't really enjoying myself. There are plenty of quests but very little variety in them past a couple of real fun ones in the starter zones.