It depends what your looking for but in general both sites will have something on offer. At the adult sites you can expect to find sex and erotic fantasy, lots of gay, lesbian and bisexual type members as well as swinger couples. At the mainstream sites you can expect to find sex and relationship types. It is less common to find a serious relationship on a full blown adult site but we do see some people looking…
The point is there is cross over from one to the other, I’ve seen women looking for relationships - long term type on adult sites and I just think no way your in the wrong place. Your going to get hammered with replies from guys who are just after a casuald fling every time so you must be kidding yourself. In contrast you do see quite a few girls advertise for guys and making a point of “not looking for anything serious or long term” It’s not uncommon.
The other shift we are seeing is the community nature of the sites coming into play, more and more people are using a website as a hangout rather than getting out to a regualr venue. Not sure how healthy this is but it is happening and as the new generation comes through it will only become more common. We may see a further fragmentation of the “dating industry” into niche groups of people with like minded interests.
So the question is pretty easy to answer - the adult site is going to be more of an adventure ride and the mainstream sites are more community contact find true love and romance. At the end of the day it is yoru choice as long as you go in knowing what to expect from each site. One thing is for sure your going to find lots of grey areas in between.