I've used WS_FTP since 1999 (started with the LE version and now using Professional), trying others over the years just to check them out, but always hold onto WS_FTP. SmartFTP came closest to converting me, but it had the annoying trait of compressing all my files when I downloaded them from the server to my local machine. Whether that only happened to me or not, I don't know, but it wasn't acceptable, so I'm sticking with what's worked for 12 years.
I'd highly suggest you trial FlashFXP, best FTP software I've used my whole life, for years now. Tried many many and none got to be as effective, efficient and stable as FlashFXP. It's the only Windows program I've ever purchased... hehe.
I use WinSCP, putty, and pageant to control public/private keys with them. I used to use FileZilla until I found out that WinSCP can be utilized in a batch mode to get nightly backups from files. I don't know if FileZilla can do that, but it was easy to setup with WinSCP