I guess it would depend on how old you are and how long you plan on staying in the house.
Young and not so long, ehh..forgo the bath. Older, or plan on staying a long time, get the bath. Let me tell you, as you get older, going up and down the stairs can be a pain in places other than just the legs. And not to get too graphic, as you age, you generally have less time after the "urge" to get where you need to be.
As a third option...don't get the full bath from the builder, but after you move in, contract with someone to put in a 1/2 bath.