The Drum Dial and the Tension Watch are pretty much the same thing. The only difference I could see were the markings on the dial. The Drum Dial indicates standard dimensions and the Tension Watch indicates metric.
I did a little interned research on these about a year ago. The way they work is there is a spring that presses down onto the surface of the drum head and measures how much "give" there is. Just like pressing your finger onto the head. The dial indicates exactly how much "give" there is. A looser head will give more and a tighter head will give less. The problem I saw is that the numbers will be different depending on what heads are being used. (i.e. singly ply versus double ply and coated versus clear) The only advantage I could see is if you had to replace a broken head in a hurry these tools will get you in the ballpark pretty quickly. I always do a quick inspection of my entire kit before I pack it up for a gig so I've never broken a head on a gig.
Here's a link to how you can build one yourself pretty cheaply: