If you have the right idea to fill into the domain, I think the domain name is the less important thing. I am not talking about domains like sex.com, or the like which by definition are important and expensive, but with the right idea you can achieve a lot even with a meaningless domain name like "youtube"
I think Site/Domain Name is more important because domain name indicates in which field you are dealing.
Both are important . if you want to buy domain then consider short and name should be your relevent industry use in Domain . like if you selling shoes then use show keyword in your Domain , About content . so Content is King .