They're two different options to download a file from MM2X. The "Download Now" button allows to download the file hosted in the main server, which uses Linux and is the one where this website is on; the "Download Mirror" button allows to download the same file hosted in the secondary server, which uses Windows and is usually faster than the main one.
Attention to some downloads that use the .7z file format. In case the download from the mirror fails, to download them, you must use only the first button. I talked about this issue right yesterday in
this topic.