WordPress Address and Site Address confusion

I am confused about whether the address format:

http://domainname.com or; http://www.domainname.com is better for SEO, or other reasons.

Currently, my install (which is the default unaltered original version) shows the http://domainname.com version in both the WordPress Address and Site Address slots under the Settings/General tab.

Having read some material on the subject I am unclear about whether to change one or both of the addresses to http://www.domainname.com, or leave them as is.

I fished around online and came across this post:

http://raihem.blogspot.com/2010/01/w…ww-to-www.html

…which seems to suggest that the http://www.domainname.com version is better.

I also discovered this post, which mentions some ghastly sounding problems that people had after changing the address(s):

http://www.blakeimeson.com/accidenta…-site-address/

At this early stage, I want to get the foundations right, even if there is a bit more work involved, however I am not clear on what is best practice. I don’t know if there are also other reasons unrelated to SEO that affect the choice.

Can anyone clarify this issue in simple terms please?

Thanks very much.