Last time I went to MP was 2007, not sure if nowadays there is more need to buy in advance. It is far convenient if you want to do the Inca Trail. Huayna Picchu section is also controled and you have to go as early as possible in the morning if you want to climb that mountain.
But since MP is one of the new7wonders, probably buying in advance assures you the entrance to the sanctuary. What I may suggest is that Master is not as popular as Visa in Peru. I mean it is well accepted in many places but Visa is accepted in almost EVERY place. Probably you may have trouble with Master sometimes. Besides the Master POS network fails often (though is doing better in the last year). Can't suggest for the online purchase but for yor travel, my suggestion is to carry 2 different cards: Visa & Master or Visa & Amex. Just in case.