If you want to play 70 - 79 for Arena's / BG's that don't pop then I'd say go to 79, but if you want to get into a BC raiding guild (lol) or want to just duel other level 70s you're better off staying at the bottom of the bracket. Since there's no more rated PvP at 70 and there won't be again the only real incentive is the community that exists FOR level 70, so in the end it's really up to what you want. I wouldn't recommend spending all your gold on a 70 twink right now since there's really not much going on in the bracket, but if you have stupid amounts of gold and can spare 30k + on gear / enchants / professions then go nuts.