Bathtub or shower, which is better for resale?

So, we’re thinking of remodeling our downstairs bathroom. It is really outdated and awful. We don’t use the tub or shower at all, so this question is not about how we will use it. But we are wondering about what would be a better value when we sell the house, a simple shower, or a shower over bathtub. We are especially interested in hearing from real estate or interior design people, if there are any such jellies out there.

A bit more background seems to be in order. The house is two-story, built in 1987, in the San Francisco bay area. There is a small bedroom on the ground floor, and the bathroom we want to remodel is across a small narrow hall from it. This bedroom is appropriate for guests, for an older person like a MIL or grandmother, for an office, or for an exercise room. All the other bedrooms are upstairs, and there is already a bathtub upstairs.

So, what do you think people would prefer, a shower or a tub?