If the millesecond that it takes to move your hand from the joystick to the 3rd SCV control is going to make a difference in your life, then spend roughly 3 to 5 times as much money, and get a joystick mounted diverter switch. Personally, I would have to put together a whole lot of milleseconds to justify $1,500 compared to $350.
Here's a scenario I envisioned when I was making the same decision -- how often is the diverter switched so that when you move the stick to perform an operation (let's say a dump) it does something unexpected (like opening the grapple)? Also, how much time are you going to spend trying to remember whether after you throw the switch, the grapple opens and closes on the forward/back or the right/left? I guess I am old enough that, not growing up with a Nintendo game controller in my hand, I would just as soon have a separate control.
Now, if you want to spend some money that is really going to make a difference, get the forward plumbing kit for your 3rd SCV. I have not done that yet, and I can assure you that I would rather have hard line plumbing on my loader for the grapple (instead of flex hose running to the back) than an electric diverter switch.