I've corrected my site with xHTML transitional validation check using validator.w3.org but after sometime, my one friend suggested me to improve my site score for HTML5 validation too. he said that it works well now for many sites and good to rank in Google too ! Is that true ? that to get a good #1-#10 rank in Google, we must have a valid HTML5 coding on our site? So, Google consider this or not ? Which one i may choose ? xHTML Transitional or HTML5 ?
Your friend doesn't know what he is talking about. Only use html5 doctype if you are actually using html5 elements. And why are you using xthml transitional? This isn't 1997 - if you are still using deprecated code, you should consider updating your knowledge or a choose another job. Use XHTML1.0 strict (or HTML 4.01 strict)
Actually i am using one WordPress theme, that have xHTML transitional declaration in <html> tag. by the way, i know HTML5 very well, that's why i have confusion about that.
I've never heard anything about google preferring html5 sites but there is no real reason not to use an html5 doctype unless you have a legacy need.