Watts vs Volts is very simple:
Think of watts like the power (which is converted into heat via the atty) that is going to be passing though the coils. In reality, with a regulated mod, this number is the most important. More watts, hotter it gets. However, things aren't that simple: you have this intermediate number (volts) that has to be drawn though the battery into the atty. The watts is a function between volts and resistance (measured in ohms). The higher the resistance of your atty, the more volts need to be pushed into it to get the wattage you want.
So, let's say you want to vape at 20 watts, and you have a 2.0 ohm atty. Your mod has to do some math to figure out how much voltage to draw from the battery into your atty to achieve the wattage you want. If you instead have a 1.0 ohm atty, your mod would draw less voltage to get that same 20 watts. Have a 2.5 ohm atty? Your mod would have to draw more.
So your mod is measuring the resistance of your atty and figuring out the volts to get the wattage that you want. I don't know the math off the top of my head, but that's the basic principle.
If your mod supports vaping on watts, there is no reason to directly control volts. Just figure out what wattage your juice is happy at, and set your mod to that - then let it figure out how many volts to draw.
EDIT:
When I used my MVP 2 I was pretty happy at 11 watts. For a few weeks. Then I got a Hanna Modz (DNA 30 chip - it goes up to 30 watts), and I was pretty happy at 20 though 26 watts. Now I have my IPV 2, and though I still vape around the mid 20's, being able to get a hotter hit off of it (30 - 34) is actually nice, and I occasionally set it up there.
Do you *neeeeeeeed* more than 15 watts? Probably not on a tank. In fact, those attys are such high resistance, that you may not want to go that high (I'm unsure of their safe capacity; someone with more knowledge than me should answer that). When you get to rebuildables though, 25 though 35 is a very nice range.
And one day, when I have a 100 watt box for some reason, I bet I'll be happy at ~ 80 watts