What to choose SAN or NAS ?

Believe me,  there is no thumb rule for selection criteria. Lot of factors required to take this decision. 

NAS low cost implementation, High performance is not required eg. You tube streaming,  Sequential Data access, OLTP SAN . NAS is extremely popular in ESX (VMware environments .. I’ll explain in my next blog.. keep reading :) 

Some systems can handle multiple clients access to same storage device (e.g. disk) (designed for this type of access) like VMware vmfs, but most (both server and client) systems are designed to be singe-access to the file-systems on a block-level => if there is only a slight chance of duplicate access you would really want to use some file-level access, which can be handled in these systems (multiuser) => NAS

With their own implications (largely performance wise). NAS is generally a much better way also from SECURITY. You can not lock a client from looking at ALL data in a SAN if he can connect (he has block access), but a NAS can enforce file level security.

The SAN solutions are used when the key factors are reliability, fault-tolerance, and high performance.

SAN issues - When using SAN the OS and Files system needs to be aware that changes to the blocks can be made by several systems at one, creating possible problems on the file system.