I believe that the answer would depend on how much speed you need. Most NAS that I have seen are copper ethernet and you can get up to GB speed. whereas a SAN can be ordered with FibreChannel connections to have fiber speed to the SAN itself which can make it much faster than GB speeds depending on how much money you want to spend. How mission critical are the apps you are going to be running? That can also answer your question but it usually depends on your finances.
another question is how much storage do you need? A SAN scales to many terabytes easily whereas if you used NAS you would have many NAS devices added to your network and once you started to scale up this would not be practical.
— Yuval —
A big difference betweens SAN and NAS is that a NAS provides its own filesystem, while a SAN provides block-level access. This makes it better for things like databases. SAN also allows you to do DR and load balancing by moving the VMs from server to server as needed. There’s a bit more on the overall benefits of SANs at SearchStorageChannel.com’s Hot Spot SAN tutorial (there’s also a third part, going over SAN features and implementation details, that should be up in a couple days).