google like just chose one.
i usual chose www

Either is good. I prefer without www as its easier and shorter.

I have found no difference as to whether your links include the www or not. In Google webmasters you can set the preferred choice and I guess if you go for the one without www then really your page links should also omit the www.

if you use adsense in your site, google assume this site with www and no www is different. And you need to listing both at adsense

I usually use www