www vs non-www ???
 

1.In Google Webmaster tool - Which domain prefer?? www or non www
2.Suppose if we prefer non www & can we upload sitemap for www and non www?

I have wondered myself about this when i bought a new domain to add into host, after long time with SEO, i just find out that no difference between www and non-www. Both of its performance are ok.
But www looks more professional.

The 'www' is considered a subdomain, and without it is the root domain.
For example: 'www.example.com' is a subdomain of the root, 'example.com'

The benefit of using 'www' over the root is that when you get links to your site as 'www.example.com' then you can set up a separate subdomain that may not be relevant, such as a different language, like 'en.example.com'

Wikipedia is an example of a large site that uses this structure.

As www is considered as a sub-domain of the root domain it is recommended that you use
non-www version as your preferred version. But before setting your preferred domain you need to verify both sites www and "non-www" in webmaster tool.