Although I have seen some exceptions, generally speaking, using a phrase (also referred to as long tail keywords) is better than choosing individual keywords in a search campaign. I like to use the phrase “Hawaii Family Vacation” as an example. To begin with, if this is what you are selling, then it’s a precise description for people searching for it. It is highly likely that people looking for a family vacation in Hawaii will use at least two if not all the words in that phrase for their search. If you broke them up into separate words you are more likely to get people looking for off-target things such as the University of Hawaii, Family services and vacation packages to anywhere in the world. Consequently, your search terms could reach millions of nonqualified people.
You will reach many more people with single words because of the broad applications that can be made. However, the overwhelming majority will not be interested in your offer because they’re looking for something else. You will reach many people, get many clicks and receive very few conversions.
A rule of thumb is that the longer the phrase, the fewer click opportunities there are. But, the likelihood is higher that each click you get is a qualified customer ready to make a buying decision. Another benefit is that long keyword phrases are less expensive to buy. So, you end up getting an increased conversion rate for less money.