RWS and Diana are one and the same. Unfortunately, some of them are actually made in China nowadays, and have sometimes had fit and finish issues, but are generally of a much higher standard than Crosman / Benjamin / Gamo, etc. which are all made in China.
Hard to compare the 350 magnum to my FX. They are very different beasts. The Diana needs to be heavier, as springers have a very odd recoil which takes a lot of practice to get better at dealing with. Typically, people shooting springers will end up either using the artillery hold, or something else that helps "rest" the gun in your hands and allows it to recoil, giving you better consistency. I find springers quite difficult to shoot, and that's why I went to the "dark side" (PCP rifles).
Here is my review of my FX that I did on the forum here:
https://www.canadiangunnutz.com/forum/showthread.php/1559743-FX-Streamline-Review
It should answer all the questions you have.