The "WWW" will not afect your Adsense as i know in some cases you can get page impressions for the "WWW" and for the non "WWW" site, but at the end all count to the same. This is not a problem, and as i say before is just in some cases.
For seo purposes you need to link all the time to the same address structure with or without "WWW".
Also you can use rel=canonical wich solve that issue among other issues. That comes in your Blogger blog as default.
Some times some webmasters have two diferent sites under the same domain name, one site is visible on the "WWW" url and the other is in the same domain but without the "WWW"
For Blogger all the url's are without "WWW"
http://somename.blogspot.com if you put the "WWW" as in
http://www.somename.blogspot.com you will get a error since the Blogger server is set up to work with the non "WWW" way.
Also By Definition:
In the Domain Name System (DNS) hierarchy, a subdomain is a domain that is part of a larger domain. For example:
http://www.example.com is a domain name A subdomain is anything that replaces the www in a domain name. support.example.com is a subdomain.
http://www.support.example is not a subdomain.
If you get your own domain name you can set up your domain to "WWW" or non "WWW" from your domain name registrar, Control Panel, from the website from where you get a domain name or with a .htaccess file in your hosting.
If you need more about the link cannonical here is Matt Cutts with a god explination
http://www.youtube.com/watch?v=Cm9onOGTgeMIf you ask me what is the best to use i will say use the "WWW" way.
The new generations of internet users will understand that a website can be "WWW" and or non "WWW" While old generations will all the time expect that a website need to start with WWW, the use or not can create confusion among internet users that aren't up to day with new tecnologies or just simply don't what to know.
In my opinion any webmaster should put in place an automatic way to redirect users from one version of the address to the other if your site is non "WWW" redirect all the users that type the "WWW" version in their browsers and viceversa.