(Wow, why did it take me so long to pipe up in this thread?)
I own D20 CoC and enjoy it but honestly I'd rather use BRP for CoC. Mostly because I've been playing it since the early 80's and know the rules inside and out. It's just how my mind works when thinking of CoC. That said, I 've always felt rules were kind of secondary as long as you had good players and a good GM. I was somewhat sad there wasn't a D20 CoC line not because of the rules but because my attitude is one of mo' Lovecraft mo' betta. Regardless of rule system. I mean conversion from D20 to BRP isn't exactly rocket science. It's pretty easy to wing, IME.