Overall, blockbuster and netflix are very similar in most important respects. If you are mostly a "new releases" DVD renter, then blockbuster is probably best because it is cheaper and gives you the 2 free in-store rentals per month.
I have been using netflix for two years and absolutely love it. They also just started a "friend recommendation" feature where you can recommend movies to your friends with netflix. Nice way to share good obscure movies that you find. They also allow multiple people in the family to create their own list of movies. Nice if you have kids.
If I drop a movie in the mail on monday, I usually receive the next one by wednesday. But that probably depends on how close you are to the distribution center, regardless of the service you are using.
Personally I have not found a reason to switch to blockbuster and have not tried it out yet. Netflix definitely has a better selection. They aren't afraid to offer all sorts of movies. My litmus test is the movie "Caligula". While I have never seen it (honestly! although my friends say I should...), if the service doesn't offer it then, IMHO they are a company that tries to enforce their level of morality on everyone. Walmart is really bad about having "cleansed" versions of movies and not offering books/movies/CDs that they feel are inappropriate. But that is just my personal philosophy and has nothing to do with the quality of the overall service.
Either way, the movie-by-mail experience is definitely worth it. Even if you don't rent enough to cover the cost, the convenience is still worth it.