I would say the separate shower, but the renovation will also be incrementally more expensive.

"Neither option is going to return 100% "

Neither will return even close to 50 cents on the dollar.

millworkman

"Neither will return even close to 50 cents on the dollar."

True but depending on the area and type of renovation actual return can vary a bit unless someone is a professional flipper and having work done at a wholesale rate. My old boss was a flipper before that was a term - he had a realtor's license and found completely undervalued property. He had a construction crew and his wife was an interior designer so they made a fair amount of profit this way since he could renovate for far less than most individuals.

If you have another tub somewhere else in the house and you don't personally use the tub, I'd change the tub combo to a stall shower and add the second sink.

http://www.remodeling.hw.net/cost-vs-value/2018/

http://starcraftcustombuilders.com/bath.design.rules.htm#.WuxrHK9OnYX

https://www.ceramictilefoundation.org/homeowners-guide-to-hiring-qualified-tile-installer

https://www.curbed.com/2018/3/7/17087588/home-renovation-unnecessary-mcmansion-hell-wagner

If by "greater value" you mean for resale, do nothing.
If you mean improved useage, for yourself, the answer is the shower in all likelihood.