Damn, this is hard. It's like asking which one of your kids you love more. I enjoy both of these shows equally because they both share great characters, great writing, memorable settings, and both overall just entertain me. Friends did last a season longer, but it did have it's flaws. My main problems with Friends were the characters of Joey and Phoebe. They never stayed constant and were sometimes way too ridiculous. Not that I didn't enjoy their characters, I just feel like they were poorly handled. Seinfeld on the other hand, is just way more iconic. It had a bigger impact on both pulp culture and me, personally. I sometimes find myself quoting the show on a daily basis or referencing it. I also really liked how Seinfeld never relied on changing their formula to spice the show up like most shows do. The show was already fantastically written to not need those "serious/dramatic" episodes. I guess I'd say Seinfeld is the better show, mainly because it was unique and just outright memorable in every way.
Anyway, these pretzels are making me thirsty. So if you don't mind, I'll be leaving now.