I have both a gyro (DynaGrip Pro) and a Grip Master (light and heavy strengths). There are pros and cons to each.
The DyanGrip Pro (which is only $25 at their online store) is good. It works the forearm pretty well and the overall endurance of your grip. The only downside is that it is not really quiet. It doesn't make a lot of noise, by any means, but there is a mechanism spinning, so it isn't silent. I haven't noticed any change in how well it's worked or its power over the 4+ years that I've had it. So long as you know how to keep the forces spinning at a high rate, it works great. These are good if you want a strong grip that you can hold a long time.
The grip master (and other similar products, which is only $15 at REI) is alright. It definitely works out the individual finger strength. In my opinion, it doesn’t work the forearm very much or the endurance of your grip. On the plus side, these are relatively silent (well, except for the slight noise from the spring, but is minimal) and are inexpensive. These are good if you want to have a stronger grip or individually strong fingers.
Schism gave an accurate description of these, but I thought I’d throw in my 2 cents.
To reference your personal statements:
1) I think the gryo would help more for tennis, because of gripping the racket tightly for so long… though I don’t know if you’d need that, if your hands are never sore after playing.
2) I am also not sure you need to spend the $100+ dollars on the metal line with crazy Torque. But, again, I don’t know your hand strength.
3) Finally, to reiterate what Rich said, I’ve seen these at plenty of sporting goods stores at reasonable prices.
Hope this helps.