for my taste most shonen-ai are a little too childish. They're sweet, but I hate to get over a whole story with just one kiss or a hug. That would be a relationship as I had it in primary school... every now and than it's ok.
Yaoi is much more juicy, there a lot more scenes making your heartbeat rise. I defenitly prefer Yaoi!
haha I'm so used to yaoi that it's kinda disappointing to see just shounen-ai sometimes. x) But it really depends. There are really good moments sometimes, but they have to be really well carried out to create that crazy sweet atmosphere. I'm strangely fond of gags so I'm okay with that, at least.
Loveless is alright, though it's not really my favorite. Anime.. Kyou Kara Maou is awesome for the shonen-ai boy-chase-boy that doesn't even develop well. lol
Yaoi.
Takes a lot to get me into a shounen-ai series.
I prefer either yaoi or anime/manga that is REALLY slashable (KHR! for example). Shounen-ai can be a bit dissapointing.