Don't think of it so much NAS vs. SAN, think more along the lines of file-level vs. block-level.
Some apps NEED block-level access to data, usually databases do, plus anything that assumes it has exclusive access to the disk such as MS Clustering & VMWare ESX. Often web apps don't need this level of access and are happy with purely file-level access - but it's worth knowing that anything that's happy with file-level is almost always sure to work fine with block-level too. It's this requirement that defines your access type, block-level=Fibre-Channel & iSCSI, file-level=CIFS/SMB, HTTP, & NFS (plus one or two others).
Now onto the second requirement - speed. File-level boxes usually talk via 1Gbps Ethernet (often using multiple ports) with newer, more expensive boxes offering 10Gbps ports. Block-level boxes almost always support 1Gbps and/or 1/2/4Gbps Fibre-Channel with 10Gbps and/or 8/10Gbps becoming more prevalent - they are also generally designed for higher performance under high load than file-level boxes, so take that into consideration.
As a general ruleset; if cost is the issue you may be forced to use a file-level 1Gbps NAS box, if performance is the issue then you may need to go with a top-end block-level FC SAN - if you're looking for bang-for-the-buck you have a wealth of options but may end up focussing on top-end file-level 10Gbps NAS boxes or lower-end block-level 4/8Gbps SAN boxes.
Hope this helps.