I use Netflix. Selection is amazing. They have nearly everything ever published on DVD. Like 20,000 titles to choose from or something crazy like that. Shipping is really fast because they have many distribution centers. They don't charge for lost DVDs (unless you make a habit out of it).
On the other hand, Blockbuster gives you two brick-and-morter rentals per month which is really nice for when you suddenly get in the mood to see some specific movie (so long as it isn't remotely obscure). Plus, from what I understand, you can rent games too with those free brick-and-morter rentals which is a pretty big plus.
I've never used Blockbuster.com, but from what I've read they aren't as fast as Netflix as far as shipping and turnaround times on average, but that also depends on how far you are from a distribution center.
Oh yeah....and I use an Xbox, a Toshiba and a Sampo DVD player, though I have no idea why that matters.
Oh yeah....again....when you get a scratched/unplayable disc from an online rental store it is SOOOO much worse than B&M cos it'll be days before you can actually watch the movie, rather than just having the hastle of jumping in the car and zipping over to the store. Sometimes you'll be 4/5 the way finished watching a movie when it suddenly freezes up and trust me.....it's infuriating.