After the discussion of the heavy duty Knew Concepts fret saw, I decided to purchase one. I have the previous model and will use the two saws in my dovetails class.
The heavy duty one is larger because the part of the saw that you use to attach the saw blade has to protrude from the frame so that it can be rotated 360 degrees. You have to push the mechanism back to rotate it, which means you have to release the tension in order to be able to turn the blade.
Here's a picture of the two saws, the older one on top and the heavy duty one at the bottom.
2018-03-19-Knew-Concepts01.jpg
You can see in this next picture, where i laid the old one on top of the new one, that the thumb screws you use to attach the blade are aligned, but the frame of the heavy duty is wider.
2018-03-19-Knew-Concepts02.jpg
As far as use, they're pretty much the same. I did some sawing with one and then with the other and I couldn't tell any difference. The blade is about the same tension on both.
The extra weight of the heavy duty one did not affect me at all. I didn't really notice it in use.
For the small difference in price, I suppose I'd recommend the heavy duty one just because it can rotate 360 degrees, although I don't expect to need that capability.
Mike