Depends which one do you prefer to use.

If you plan to resell one day most buyers want a tub

As long as you have another bathroom in the house with a tub you can go with just a shower stall in your MB. If you don't then you will have to have a tub in the MB.

I can't help with your decision, but I believe you are considering a "walk in" shower.

When we remodeled our MB we just put a shower in that included a seat for my needs. Also 2 separate heads which we really love. We have the same arrangement you have with a full bath on the same floor. If you have ample space I suppose you could put a tub in but you'd still have to have the shower option in there I would think. Since you have a full bath on the same floor, though I don't think you'd miss not having one in the MB.