My understanding is that you get an ITIN if you aren't resident in the US at the time you need it (i.e. filing your taxes), or an SSN if you are resident. I got an ITIN in 1997 to file taxes, but in the mid-2000s had to get an SSN to file because I was resident in the US at the time. At least, that was the advice I was given.

Getting either is very easy. Unsurprisingly, the government makes it as easy as possible to get the identification you need to file taxes and pay them money. I got the ITIN at the US embassy in Toronto, and the SSN at a Social Security office.

http://www.irs.gov/Individuals/General-ITIN-Information