If it were my project, I'd go with a big shower. I have a shower and a large 'garden' tub in my master bath and in the 2 years I've been in my home, I've only taken maybe 3-4 baths in it. I much prefer showers.
I think parents with children usually have the kids share a bathroom (at least that's what happened when I grew up...not that many years ago). ;-) If I were buying a house I don't think having only one tub would make that big a difference to me. Just my humble opinion. Melissa May