Logically, grinding is less "buggy" because all you're doing is standing there killing mobs in a small area rather than running from place to place talking to quest givers, using items, talking to other quest mobs, etc.
But as everyone has mentioned, grinding is also the more dangerous of the 2 (at least in some opinions) since most people don't really like to stand there killing mobs over and over on a daily basis. When I leveled up my last horde character, Azenius was a pretty clean set of profiles. I could actually leave that running overnight and wake up in the morning and it would still be chugging along. I also like that they split the profiles up by level range/zone so you don't have to sit there for 10 minutes waiting for it to wade through quest completion code before it starts where you left off. I tend to edit any questing profile I download to split them into different xml files now because of that.