same

They are both fine, but in formal situations (such as a job interview) I would stray away from using pronouns. If somebody asked me "Do you have experience in sales?" I would say "Yes, I have experience selling cars." This is more of an interview tip than a grammar correction tho, haha. Hope this helps :)

depends on what you say before it. If it's for a resume I would just say I have two years experience in (adding the subject area here). Another example would be, if someone says I am looking at finding someone with ... skills. You would say I have 2 years experience in that.

They're both correct -- "that" is more emphatic than "it." you could also say "I have two years' experience." Note the genitive. Note that numbers at the beginning of a sentence or below ten are preferably spelled out.