Hello Joel.
I have some objective data based on our testing here. Over the past 18 months, Barracuda's block rate is 81.9%, while Spamhaus' is 83.3%. For whatever measurement error you want to include, that says that they are roughly equivalent. Over the past 6 months, BRBL is actually getting better: their block rate is 87%, while Spamhaus is 82%.
There is, of course, a catch. BRBL gets a higher rate, but at a substantially higher false positive (FP) rate. We normalize FPs per 10,000 messages our measurements. Over the last 18 months, BRBL was 4.1 FP/10K messages; Spamhaus 0.2 FP/10K messages. Again, BRBL is getting better: over the past 6 months, BRBL went down to 1.6 FP/10K messages, while Spamhaus is about the same at 0.3 FP/10K messages.
Your numbers reflect what I see, too. One other thing to note is that the two services don't catch exactly the same spam, so using both results in better trapping than either one alone. John John Souvestre - New Orleans LA