Okay, first, I'm a Google fanboy. I use Google mail, search, Earth, Blogger, and (until now) Google Maps. But as a result of playing around with Yahoo Maps (as a result of placing my Flickr photos onto maps), I'm really thinking Yahoo's winning my heart for online maps.
Both Yahoo & Google maps let you type in a zip and get there quickly. But Yahoo's maps have some neat extras. 1) They mark county lines. 2) They have townships more clearly marked. (I think Google has these, but they're not as obvious.) 3) The most cool thing: when you pull up a map, you can enter a type of business and see all of that type of business on that map. (After playing around, I think Google maps will do something similar, but it's extra clicks away, and usually takes you away from the current map when you click it.)
In typing this up, and making sure I've given Google a fair shake, the two sites aren't as different as I was initially thinking, but I'll still give kudos to Yahoo Maps for making locating a business feel much easier (even if it's only a little bit easier) than Google Maps. (And as a dig to both, both had the pharmacy closest to my house, but neither of them had it placed on the map anywhere NEAR where it actually is.)