What are the advantages of using Secp256k1? Bitcoin uses secp256k1 as the specification for it's address system (https://en.bitcoin.it/wiki/Secp256k1). What are the advantages (and disadvantages) of using this over other specifications such as secp256r1?

Edit: I found this page (https://bitcointalk.org/index.php?topic=151120.0) which speculates on some possible differences, but I'm hoping to find something concrete.
I believe Satoshi was once asked this question, and answered that it was just some curve that existed and was presumed to be efficiently implemented.

I'll try to find a reference.