As a client between the two, I prefer P411. A provider "gave" me a "temp" membership to DC, and I found the site to be less than user friendly. Also, I thought it was more of a board like ECCIE than a verification/screening site.
P411 on the other hand seems to be a straight forward verification site with some added features for locating a provider. I tend to think the providers who use P411 are a little more cautious than other sites. I like that. I also like the fact that providers who use P411 don't require all the BS info usually needed for screening because P411 already has the info there for the provider to use. On the other hand, there are 2 things I don't like about P411 (one of which is probably unavoidable). I think $90 is a bit pricey. I also don't like that they reveal my name to the provider; although; at some point, that info has to get into her hands. The reason I don't like providers knowing my real name is that they contact me using my real name. I'd simply prefer if when being contacted by a provider that she use my handle.
As for advertising sites, I think E.Com is pretty much universal. ECCIE is wonderful for Texas. I've met several providers off of Indys (Ohio). Out in Colorado, there is The Other Board. Supposedly the New Sweet Dixie is for the SE. As for BP and CL, there is some usefulness to those sites, but nobody should use them exclusively for booking a provider. I'd say 80% of those that advertise on BP/CL are not worth it (one way or the other).