Having a cool short name as well as a keyword rich name all pointing to the same site would be ideal I think.
Also, a cool short name can also work okay for SEO if you are targetting only mildly competitive keyphrases so long as page names and folders are keyword rich.
ex) http://www.coolsite.com/big-red-tractors.html would likely do pretty well for a search under “big red tractors” so long as the page was optimized well.
The best approach may be to do various searches on the Search Engines you will be optimizing for and see who is ranking well. If only keyword rich domains are getting top ranking then you may need to do likewise. If there are numerous sites with “cool names” that are still doing well then maybe you wont need the keyword rich domain. It all depends on how competitive the keywords are.
Study the competition and emulate what they are doing…