A nice custom shower in the master is the way to go. As far as resale is concerned you always want to make sure you have at least one tub in the home (for babies and children). you have more than one so you should not be concerned about removing the tub from the master.

Ask yourself if you would actually use it enough to give up a bigger shower. I myself wouldn't because I just feel a shower n storage are a better option if I had to choose.

With that many other bath tubs in the house I would go for the option with more storage!

Master bath gets a shower alcove cause this isn't the 80's, but every house needs one tub minimum for those long days