I've always done both since 2003 and the reason that if one area has a dip doesn't phase me much as usually something else is running strong.
If this was 2003 I'd probably say Adult as was normal back then to see conversion ratios of 1-x, 1-xx and 1-xxx where today normal leans more to 1-xxx, 1-xxxx as free porn galore as the game has changed to more of a give it away and sell ad spots. Had sites back then with only a few hundred visitors a day making bank.
I'd probably lean to mainstream today and not because I'm not making money in Adult anymore but because you don't have the adult restrictions so if you are strictly talking from an affiliate angle can promote mainstream shit anywhere and personally I make hundreds of dollars on some single leads I sell in mainstream.
Would still recommend doing what I did though. Run your adult game and tinker with mainstream on the side until you figure shit out and run both so you have a diversified income stream.