1. If you are downloading on mainframe
Maybe I'm missing what you mean by this, but I think all mainframes could serve both FTP and HTTP, so that's not much of a difference.
2. Using FTP you can write a script and then schedule it to run per month, is this right?
Sure you can, but you can do the same for HTTP.
I think I'd base it on what level of authentication is required. Setting up multiple user accounts with their respective permissions is likely to be more of a hassle for HTTP than FTP. If, on the other hand, this is publicly available stuff with no authentication, I'd go with HTTP.