rentacar.com is not better than rent-a-car.com as far as keyword and Search engine optimization is concerned.
The keyword rentacar in google gives: 214000 listings
whereas
the keyword rent-a-car gives
2760000 listings
and
the keyword rent a car gives
8300000 listings.
When people types rent a car
google considers it as rent-a-car
and not as rent car even though it ignores the letter "a".(http://www.google.com/search?q=rent a car)
So rent-a-car.com is more valuable than rentacar.com for SE optimization