Will it effect resale if I don't include bathtub in master ensuite?

Personally I don't think so. We see more and more homes where the family bathroom has a bathtub, the other bathrooms have showers only.

As long as you have a tub in home (surprising how often they are needed when you do not have one), I would do what you prefer. Many people often discuss resale. My husband and I typically build "for ourselves" b/c whenever we have sold a home, people will always posture regardless (no double ovens, backyard too small, etc). You can't please everyone.

One tip, one of our homes has a range of shower heads that can be handheld. Low and high. The main shower head is not removable. It has been easier for me to use these in our large shower when washing my youngest daughters hair. Typically, i would throw her in the tub but with multiple shower heads at different heights, I use whichever sprayer works. They all have a separate knob for turning on/off. I am shocked how I have not used our tub b/c the options in the large shower are better.

If you have a bathtub somewhere else then I'd say please yourselves and go ahead with that big shower you're dreaming of. @dhamda - great tip on using several showerheads, or if you're going to use a single shower head, make it height adjustable and detachable so that it can be hand held.
Another tip - I've seen luxuriously finished showers spoiled because there wasn't enough thought put into the detailed design. Think of things like... Can I turn the shower on without getting my sleeve wet? Where will I hang my towel so that I can reach it from the shower or shower mat and not leave a dripping trail on the floor? How will I store the shampoo and other bottles so that I don't have a cleaning nightmare on my hands? You get the idea - get down to the functional detail to make sure your design is a success.
Best of luck with your remodel.
Meg - the House Plans Helper.

As long as the shower in the master ensuite is designed 'well', potential buyers shouldn't mind not having a tub.

Like others said, if you already have a bathtub in the house, go with the big shower! We are seeing more and more large showers in bathrooms. They can make a bathroom feel so much bigger and cleaner! Good luck :)

Thank you all very much great tips!