What size PEX for this project?

I'm doing a new shower and tub adjacent to each other and will be running PEX for the project. The question is I've heard some people successfully use PEX for tubs IF they use 3/4"... and that using 1/2" is a problem when used in combination with a diverter and shower head because the water can back up and start dribbling out the shower head. But what about for this type of scenario? The tub will be a separate clawfoot tub next to the shower. Does this eliminate the aforementioned problem? Or do I still need to go to 3/4"? And if so, do I need 3/4" going back to the shut-off valve also? Before the shut-off valve is copper, BTW.