Some like one verison or the other. Sometimes depending on the setup non http can serve up pages slightly faster due to look ups. But the internet is so fast so it really doesn't matter to much except in extreme cases of poor conectivity or poor dns.
It's really a matter of preference. If you notice all the big boys, Google, Facebook, Twitter all use the www URL prefix. They must know something we don't? At any rate I prefer to use the www especially on shorter domains. People still respect the www and the search engines still do to. Not really sure of it's SEO effects but I use it always. A word to the wise, if you choose the www then stick with it always. Never backlink without it because believe it or not it can cause duplicate content. If Google has indexed the non www and then you switch to the www without removing the non in webmaster tools and using a 301 redirect it can cause issues. Hope this helps.
What is nowww.?? do you mean difference between www.abc.com? and http://abc.com? check this threads: http://forums.digitalpoint.com/showthread.php?t=183636 Or http://forums.digitalpoint.com/showthread.php?t=409810