Because apparently at some point the question has to be asked, even if it is almost like comparing apples to oranges. Sure, both are supernatural-type shows, but that's about the only real way they are alike. Still for me the simple answer is that Supernatural is the better show. Of course, that isn't exactly fair for many reasons, but especially because Supernatural has not finished its run like Buffy... Unless you count the fact that Whedon IS continuing the BuffyVerse in comic form. *coughs*

In truth, though, for me Supernatural and Buffy are pretty much tied as best for me, but right now at the moment I'm really enjoying Supernatural and keep watching the DVDs of that instead of pulling out the Buffy DVDs. I am, however, loving season five of Supernatural only about a million times more than I liked season five of Buffy. Truth be told there's only about six episodes from Buffy's season five that I do like, and I think season five (of Buffy) contains the worst plotted arc of Buffy's entire run. Yeah, that's right, I think season six was better than season five, and while I did NOT like the way seven ended (I liked Chosen... Well, mostly. Okay, not really, but it didn't suck as bad as the last block of season seven episodes sucked. So, yeah, I HATED everything leading up to Chosen, and wasn't real fond of the very last episode itself), but the overall season was still better plotted than season five. Not super awesome, mind you, but better then five at any rate.
PS - Just so you know while I think Angel (as much as it pains me to say that) is a better show than Buffy at least continuity and plot-wise (though it made its mistakes over the years) I actually like Buffy better than Angel. Just so you know how things stack up for me.
Just cause it's "better" doesn't mean I'll like it moar!

So where did this come from? Well, when I was cruising Whedonesque for the Twilightgate links I came across this very question, and while I tried to ignore it at first, because I don't really think it's a fair question, because it many ways the answer comes down to a matter of taste. However, obviously I could NOT stop thinking about the question.

For the record, when Buffy was good she was VERY good (as in ten times better than the best Supernatural episode). The problem is that when she was bad, she was freakin' horrible and nigh unwatchable. So, while Supernatural doesn't have such lofty highs they also don't have the horrible horrible lows either.
Out of four seasons there's only been one Supernatural episode that really truly sucked for me (season three's Ghostfacers). Where as in four seasons Buffy had at least four (maybe even five) episodes that truly sucked for me.

Also, and again this comes down to a matter of personal likes - Whedon did many things over the years with Buffy that I truly hated, and while Supernatural has had some things (characters) over the years that I didn't care for, usually Supernatural will correct itself and get rid of the things/characters that don't work for me while Whedon almost continually insists on pushing those things/characters down my throat.

I also, personally, think that Kripke is better at pulling together his overall story arcs in ways that make more sense than Whedons. Whedon relies on the emotional punch to make his work, and over time that punch usually loses its punch and when that happens there's nothing to keep you from thinking about all the ways in which it actually doesn't make sense.

Okay, yes, Kripke does have problems (sometimes) with little bits of detail, but then again so does Whedon. Biggest example is "You were my sire, man," vs "No, seriously, Drusilla is my sire." *sigh* If you don't know this, back in Buffy's season two it was revealed that Angel(us) sired (made Spike a vampire) Spike. Then in Buffy's season five Whedon changes his sire from Angel(us) to Drusilla.

However, I think Whedon is (most of the time) better than Kripke at allowing you to get lost in his story. But I think that's also Whedon's downfall, because Whedon gets you into the story emotionally, but as I said over time that emotion can fade and then all your left with are little things that don't really make a lot of sense the more you think about it.

Honestly, it's really hard to say (at least in a Tru Fax, ya'll kind of way) that one is truly better than the other, because both have their strong and weak points. Right now, yes, I think I like Supernatural better, but years (or even months) from now I might find myself wanting to watch Buffy MOAR, especially if Whedon makes another TV series in this verse - HINT HINT, Whedon!

In the end, I think which one you'll like best depends on what kind of mood you're in. If you want a big emotional story then Buffy is the show to watch. But if you're in the mood for a story that makes you go "holy crap! I did not see that coming, but that/those twists actually work!" then Supernatural is the show to watch.