Good question and you will hear many opinions. The best answers will depend on what your goal is for the domain name. I will assume you mean to buy and later sell and not just for parking.
One word domains almost always have more search results pages (SERPS). Why? Because when you search without using quotes, you are seeing pages that must have what you are looking for on the page. So if you are looking for one word, all the SERPS will have that word somewhere on the page. If you are searching for two words, then all of the SERPS must have both of those words somewhere on the page, but they don't have to be next to each other.
If you do a search for two words but put them in quotes, then the SERPS will show you all the pages that have those two words and in the same exact order you entered. These SERPs are a much more accurate indication of how much your keyword phrase is being used on web sites. But this does NOT tell you how many people are using the keyword phrase to search with, but that is a discussion for another topic. The number of searches being done is more important for domain parking.
Now you can see how you search affects what results you get. I would not buy a domain just because the SERP number is large. What is more important to me is how many ADS are showing, since I buy more for parking than for resale value.
I think a domain like MinneapolisCarAccident.com may be more valuable than a domain like Authoring.biz. While the domain is long, the words are in the proper order and match what some people search for. If the SERPs are low, that's ok because the domain and any site on it should be found more easily for the exact search phrase. This is important for parking, but also to a buyer.
Authoring.biz is not a bad domain, but how many large customers will settle for a .biz domain? Not so many I think. And because it is a .biz it suffers from a lack of respect in general like .info does. But people in Europe seem to like .info so I think it is better than .biz.
So if you can get a good targeted 2-3 word .com you should do well. After that consider getting a .net. that will be most people's second choice. Then I would look at .org, but not for all KW. Then I slightly prefer .us over .biz and .info, but there is not a lot of difference to me.
Good Luck!
