Bedroom count is very important in buyers searches. If you can can add the space for a den, allow for the buyer an option to use it as a 3rd bedroom. In my area the requirement for a "den" to be called a bedroom is the existence of a clothes closet.
This depends on where the units are located. A den can really be a plus in the resale value. This can give the next buyers some options to use this space as an office, den, second family room, guest room. You will also have to consider the cost involved to add a den. Usually the cost will be re-couped when you sell.