This is running off a Windows server, I take it? Go with Serv-U.
I tried BPFTP Server, but found that the customization was lacking in certain areas. You can't effectively share a drive and then lock out certain parts... you have to add it all manually. Also, Serv-U's CPU/Mem utilizatoin was a bit better, and has been obscenely stable for oh I don't know... 3 months on my server. I haven't rebooted yet, running Windows 2003.