Hi it depends what you want to achieve.
Currently i am working over classic modding, but checked also tbc. So i can say, if you want add more playable races chose tbc.
Classic can support only 9 races ( 8 currently avilable for players) , tbc can support about 16 ( 10 currently used) .
TBC requires wow model edit fix starterd before wow.exe if you wish to have additional patch.mpq with modded stuff, instead wow will crash after entering to modded area.
Classic and tbc requires more tools for creating own areas. Also you must remember about tbc or classic client limitations.
it was a short overwiev from my side.