Many valves have two outputs, one is smaller than the other, and since you want to fill the tub the fastest you can, you use the larger outlet for it, and the smaller one for the shower. Nothing says you can't swap them, but it often isn't necessary, since you can easily get the max for an approved showerhead out of the smaller tap. A single showerhead can use up to 2.5g. A 1/2" valve can easily flow maybe 6g (more when you add hot and cold), but exceeding that can cause excess wear from high velocity. So, most any valve could work until you start a car wash. Multiple heads also means exhausting your hot supply, or having a large one, incurring extra costs. If you don't already have 3/4" lines to the bathroom shower, you may need to run them; depends on how much volume you need to keep the pressure up with the number of heads.
Delta's 1700 series of valves are nice and use the R10000 rough-in valve. The features depend on the trim/cartridge kit you choose. And yes, when you buy from other than the big-box stores, you often end up buying the rough-in separate from the trim. You have three generic choices: generic pressure-balance, pressure balanced with volume control, or thermostatically controlled valve, all from the same rough-in. Depends on the trim package you choose. Prices go up in the order listed.
I've had good luck with my Grohe stuff, availability is good in my area, may not be where you are, and that should be a factor as anything, eventually, needs some service. I like thermostatically controlled valves, or at a minimum, a pressure balanced one with separate volume/temperature knobs. This way, you can set the temp you want easier, and it stays there. The thermostatically controlled ones work better in season changes as the cold winter water doesn't affect the output as much.