It depends on what your target renter is. If you think you can maximize rent by targeting families with young children, I would keep the tub. If you could maximize rent by targeting young professional roommates, it probably doesn't matter. If you are targeting elderly tenants, a no/low threshold shower or walk in tub is probably a great feature.
Do you know what the existing drain and vent size is? I believe (i'm not a licensed plumber) that you need 2" drain and vent piping for a shower stall, but only 1.5" for a tub. If you're not getting inspected, it probably won't matter, but if you are getting an inspection, they might make you re-vent/drain, adding potentially significant cost to your reno.
As long as there is one bathtub for small children, a shower only in the smaller bathroom would be fine. If you go with NO bathtub -- only showers in both bathrooms, you will limit your appeal to adults only renters.
Am with @Matt Leonard as relates to your target audience. I Have a basement unit with a 3/4 bathroom that works just great with millennials. currently house-hacking in this unit as I get ready to move. I have no issue at all with stall over bathtub personally and I think it really depends on your target renter.
Join the Largest Real Estate Investing Community
Basic membership is free, forever.