Alternative (secure) FTP server - G6, Serv-U, something else?

[jaytee]

We have received quite a few requests for adding a secure upload option to Plesk for Windows. Originally, most of requests were for Serv-U FTP, but recently I've seen more comments from the people who prefer G6.

Please post your thoughts on which FTP server we should choose - we would rather choose one and invest more efforts in supporting it well, than offer you a wide choice of poorly integrated solutions.

I myself would probably prefer inexpensive (so more people will be able to use it) software, ideally with SSH (SFTP/SCP) suppport. FTP over SSL is great, but sometimes has problems with firewalls.