Easily TBC. People saying you have to regrind stuff in retail are deluding themselves. To get AOTC you don't need grind anything once. You just turn up the first week. Also, with each phase better gear is released with bosses which require better gear to down. How is this different from retail? Your gear doesn't stay "relevant" the whole expansion. A few over-tuned items here or there which you keep using is no different to retail, which frequently has items which are used in later tiers because of how powerful they are.
For those who want to spend a lot of time in either, both have ways to do that. In TBC those grinds are mandatory, in retail they are optional.