VVPS changes the voltage, which changes the temperature. You don't get to see what the exact temp is, but imo. You don't need to know exactly what temp it is. With voltage control, you'll find what voltage works for you, which is effectively the same. Your preferred voltage/temp will vary depending on a number of factors including what kind of effects you're looking for and the strain/moisture level of your material. It takes some experimentation, you start at 12v and see if you need to raise or lower the temp to get what you want. As for plugging it in 24/7, really up to you and I personally think the difference is minor. Some say it's better for the wood to be always plugged in because it doesn't expand and contract, others say it's better for the heater to be off as much as possible for lifespan. I don't know who's right and don't really care because whether you leave it plugged in constantly or are diligent about turning it off, this thing is going to last a very very long time and will be inexpensive to repair if it does ever fail.