Friends is pretty easy to understand, seinfeld feels like it's somehow appealing to a smaller audience.
Friends has a bigger cast and bigger sets - Seinfeld uses a tiny, though realistic, apartment, and the diner they go to feels more closed in. I don't watch sitcoms for the realism so i think friends wins here because it's better thought out.
Seinfeld struggles to control his smile and laughing, and it makes it seem like he's a comedian constantly aware that he's just playing a character or acting out skits he wrote. I think Friends has the benefit of the actors not, usually, writing their stuff. Also in Friends they eventually developed brilliant talent at riffing/ad-libbing, something which Matt Perry demonstrated from the get go and helped carry the show with.