One valve and a diverter means that you can only have water from one shower head. A diverter is what you have with a bath tub / shower where you choose to have the water at the tub spout or shower head. I believe this would be the lower cost option.

With two valves you can have water from both shower heads simultaneously. But then, you would have to adjust the temperature of each.

How do you intend to use the two shower heads?

Hi,
We would like the option of being able to use each head separately.

There are in-line valves which when installed just behind, or prior to, the head, allow you to have either or both heads operating, while the water flow and temperature are a single adjustment.

The simplest way is to run hot and cold supply to a thermostatic mixer to regulate temperature and then route the output to two valves each controlling one shower head. With a superficial search, I did not find any Delta products that do this.

Hansgrohe can do this with one iBox in the wall and an appropriate trim package, but be prepared to spend more than that Delta shower valve.