I prefer both. The good point of R18 games is that you get to see all sides of relationship with guys without missing anything. Also, as many R18 games feature rape and abuse, I prefer them when I'm in the mood for a dark, tragic story. At the same time, as I don't really enjoy rape or objectifying a heroine by her guys, what I really want is a positive development after all her hardships. Still, what I really can't stand and what can force me to stop playing is when a heroine is totally OK with a guy who rapes and abuses her, forgiving him right away. Like it was in Akazukin to Mayoi no Mori. I stopped playing Ookami-san's and Kitsune-san's routes after such events, thinking about the heroine "Well, maybe you've forgiven him, but I haven't". No way in Hell I can associate myself with a heroine like this. I feel sick with both a heroine and her guy in such cases.
As for all-ages, I really like them when I just want to enjoy a good plot and romance without all those steamy scenes. On the downside, in some novels, when it's, like, a year has passed since a heroine and her guy started dating and she is still embarrassed to take his hand and kiss him on the cheek, I can only facepalm so hard.