For me it also depends on money which you dont mention. Follow the money. And there's the work life stuff, can you travel where are you based etc.
Don't believe the 'you'll be promoted quickly' rubbish unless explicity in your contract - very unlikely. You could find yourself:-
1. new boss who didn't make that promise (reorg, they leave etc)
2. Its competitve - how many others have been made similar promises or going for SM or they are out - no one would care about your empty promise if their careers are on the line
and finally, think of the exit situation. If after 2-3 years you leave are you better leaving with Assoc Partner on CV or Manager, remember if you did leave ACN after 2-3 years it is probably because you are still manager.