Rather than spending the money for constant pressure pumps, you can use the Smart Tee or the Cycle stop valve and do about the same thing. You can use a smaller tank just like with the constant pressure units but save a bunch of money in the process.
You can turn the pump on at 60 and off at 80 with the Smart tee set at 70 and have 70 psi constant pressure as the pump runs. Same thing with the constant pressure pumps. The pressure is only constant when the pump is running.
A pressure regulating valve is basically what the Smart Tee is.