Now that I am in the final stages of planning my trip to San Francisco, Lake Tahoe, Yosemite and Carmel, I am looking for driving directions. Is one of these mapping sites better than the other? Any other better ones? Would I do better going to the AAA?

I like mapquest, but I don't think either site is superior to an actual map, particularly for a multi-city trip such as you are planning.

I suggest getting a map and driving directions from AAA and using the online maps as backups or for short trips at your various destinations. If you click on "Yellow Pages" in mapquest, you can find lists and driving directions to such things as the nearest theater, grocery store, museum, etc, which can be useful when you're in an unfamiliar city.

I often use online maps here in my hometown(San Diego). I usually check both mapquest and maps.yahoo.com because they sometimes give different routes. Also, sometimes the routes aren't quite accurate. That's why I wouldn't rely solely on them for a trip such as yours.

my preference is freetrip.com It will also list hotels along the way.

Funny you should ask. Dateline did a piece on those mapping websites last night in conjunction w/Good Housekeeping magazine. They rated Mapquest as better than Mapblast. I think Yahoo Maps came in 2nd or 3rd. A couple of them gave ridiculously inaccurate info.

I use a combination of Rand McNally & Mapquest. Print directions & maps on both & then compare, & may use 1 or the other, or a combination of both, depending upon what I see in the output.
A year or so ago, I went to AAA (been a member for about 20 years) for a triptik, & they said they do not do door to door directions, but will do city to city, which may or may not be what you need, unless they have changed that in the last year. But the maps & other information about the areas traveling through & to, were very helpful. Besides, you don't have to pay extra if you are already an AAA member.