A sounds like the much better deal. I wouldn't count on friend taking you along for ride. Two examples off the top of my head, someone I know was like the first 5 employees and a friend of person started business, always hinted would have more of a cfo type role. However when their business expanded, hired outside, re-wrote his job description and actually makes less now and less benefits (the promise didn't pan out). Other story, friend hires a few of his friends, starts a business. Doing great, business is growing, and when it gets to a certain value friend allows business to be bought out. He is multi-millionaire, friends who are still there, get let go. I could go on, but I won't.
Besides, if he was such a good friend, wouldn't he try to hire you out from A when his next great idea comes along? What do you have to offer that you are so sure he would take you along for the ride, and be rewarded for it. Like above just "being there" doesn't usually cut it.