Start with the second half of this article to understand how they differ:
When it comes to how to evaluate, it varies by system:
Block: (Sometimes called SAN, but thats not really correct) Usually people here will focus on a couple things:
- Raw Performance: How many operations per second can I execute? Usually called ‘IOPS’, we are talking ranges of a couple thousand for low end home-type systems, all the way up to millions of operations per second for high end systems. How many you need depends on your application