I would say the best plan is to provide the potential buyer with a tub option. Even though the spa idea is very attractive to you and many, I suggest a compromise. Do install a tub (earth-tone or other spa-like color), and customize with a bench behind it, spa-like tile and fixtures (like the rain shower head, or multiple heads.)
I realize your space is limited but with a good bath/tile contractor you should be able to accomplish this. You'll get most of what you want for your current enjoyment, as well as preserving maximum appeal for resale.