Friends hasn't aged that well for a 90s sitcom (compared with Frasier, for example). HIMYM obviously doesn't have that issue right now, so it's impossible to compare them fairly. I prefer HIMYM myself, though. I can watch that show all day, but Friends gets annoying after 2-3 eps in a row (maybe it's just because Comedy Central always seems to have the same episodes on every time I watch it on there?).