I've had both good and not-so-good with both. Was with Netflix for 4 years, average ship time was 3 days round trip, though there were some flukes. Went to BBO when NF raised their prices on me, and the first 3 months were bad, but they have greatly improved, 2-3 day turnaround.
At first the inventory levels werehorrible, something like 40-70 percent of my queue was in a 'wait' status. But after a month or so, it got a lot better.
The two in-store coupons are why I'm sticking with them, at least till my price goes up [I'm at 14.99/mo now.] Two game rentals are 16 dollars, it's like I'm renting two games [for almost two weeks each] and getting free unlimited dvd rentals to boot.
They're both good, and it has been reported that both will throttle you if you're a real heavy renter.
The only thing about BBO is that you are more likely to get stuff that's not at the top of the queue--they try to ship stuff from the distro center closest to you, so it takes less time to get to/from you. It's a tradeoff, and I can see benefits doing it either way. They also ship from actual stores now, the disk I got today is from a BB store three hours from me.
I will admit, BBO's website and queue management are not as robust as Netflix's [at least based on my memory]. I don't need the 'share with a friend' features, but I would like a little more flexibility on the website.
There's no contract or obligation, pick one and try it. For the first few months with either one, I kept a spreadsheet of 'sent/received/sentback/checked in' to track speed, and I noticed a distinct improvement with BBO after 1.5 months [I also joined at Christmas, so holiday mail and holiday rush might have affected my service].
My coupons are still usable on games, although I have heard they are phasing them out [and that would cause me to revisit my loyalty to BBO]. When I signed up with BBO, there were maybe 3 titles out of 150 NF had that BBO didn't, but BBO had a few NF didn't as well.