I've used both, and Netflix was MUCH better, IMO. They were faster and the format was much more user friendly. Blockbuster's layout was a PITA, and they were slower than Netflix in sending out DVDs. I usually get my DVDs from Netflix in about 2 days after returning the ones I watched.
I don't know anything about Blockbuster online, but we use netflix and it's really great, mostly because of how fast everything is sent.
Someone told me that they built a lot of distribution centers all over the country to make sure that shipping is fast and customers don't have to wait too long - it really works because I'm always amazed at how fast my dvd's arrive. When I return a dvd, they always receive it the very next day, send out the next dvd on my list that day, and I get it the following day. So if I send it out on a Monday, I have a new dvd in my mailbox on Wednesday. I think that's a pretty good turnaround time.
I had netflix but I switched to Blockbuster. Blockbuster has a somewhat smaller selection, but I still have plenty of DVD's in my queue. I also like that blockbuster gives you 2 in-store rental coupons a month for free. This is for when I am waiting for some to arrive in the mail. I think it takes them about the same amount of time to send/recieve.