Its always better to have a shorter name. It is easier for people to remember your domain name and hence more chance of getting return visitors.
As most already have answered, short names are almost always better. The only time I think you should use longer names is if you try to target a specific keyword and only are working against visitiors that come via search engines.
Short domains are always better. But you will get more competitor. You can easily test it on google. Type on keyword see the number of results then type 2or more keywords and see the results.