Nope, it's exactly the same
It's completely up to you whether you start it from SoA or ToB....
Personally, my only advice would be to at least wait till you have done some major SoA quests and gained some levels, since Watcher's Keepcan be quite tough. I've found that the storyline of TOB is pretty fast-paced so you might want to do Watchre's Keep in SoA.
Good luck!! [img]smile.gif[/img]