Use the search function and you will find this topic addressed in the past here. Some think a student is well-served to learn to use the old "whiz wheel" E6B because they are more reliable in cockpit environments and, as you mentioned are still on the radar of some pilot examiners. Learning them is a bit of a pain but so is everything that is worth doing. You will also have to learn to use an electronic flight computer, both are part of becoming educated in aviation. I prefer the electronic one for accurate calculations, but the whiz wheel is handy for fast calculations. As far as the ultimate value of handheld flight computers, neither has very much value because all the functions they perform are covered by modern avionics systems. Learn both of them, and I recommend starting with the mechanical one since it is so inexpensive. I carry a small version that ASA sells for about $30 which is easier to carry than the ten inch version.
As far as your trouble learning the old one, that's probably because you don't have a strong understanding of what exactly it is supposed to measure, how that information is important and fits with the other parts of the flying puzzle. I was slow to get a grip on the mechanical one too, not because it is a very complicated device but because it makes you guide it to the answer more than an electronic computer does. And that's the point- one will be almost unable to use a mechanical computer unless they already know what to expect from it and have a clear understanding of how things fit together. This forces you to develop that, and the way the electronic computer spits out answers does little to enhance an understanding of basic concepts of aviation.