Here are the differences between the 2009 and the new model.
The new model has the power supply built into the computer. Bad Bad Bad. This really affects the quality of the sound. The faster processor does not help, nor more than 8GB of RAM.
The 2009 Mac mini has an external power supply that you can move far from the computer. Good, good, good. Maxing it out to 8GB of RAM and putting in a SSD will make the an excellent Music server. Opening up the computer is fairly easy with a 4" putty knife. When you do it make sure that you have 8GB of RAM installed, maybe pop in the SSD. With the RAM ($80), Fan ($50) and the SSD ($99) you will be very happy and will have saved a bunch of money. You will also have replaced many of the parts that fail.
Dave