Oh well, to me, it always depends on how I'm feeling at the moment. Usually, I come to "enjoy the art". Sometimes, I stumble upon some golden pieces in terms of storyline. Then, I judge, if they are fappable. Usually, the best plot-based hentai manga, have so big time/page distances from each H scene, that are not fappable, even though the art can be stunning; thus, I mark them, and leave them for later, when I'm feeling like it. For some reason though, those with crappy artwork, usually are totally unreadable, so plot goes on 0 level too.
If I have to put it down and choose, I'd go with the "Hentai" side. Why? Because plot can be found in non-H manga, and some amazing stories are written on a mere "ecchi" level. But the other way around, "Hentai" cannot be found anywhere else than its own category. Thus, I wouldn't miss on what is unique ( the fappability ) to gain something that I can find elsewhere ( the plot ).