To buy? Diamond here, in my opinion.
You can download Chessbase light with Fritz enginge for free here: http://www.chessbase.com/download/cblight/index.asp
If you use that free system for a while and decide you need to upgrade to Fritz 12, you can always do that.
A diamond membership here or Fritz? And also, what is the difference between the fritz sold on amazon and the one sold on chessbase.com, other than the price?