Rotational throwing is probably the easiest to learn quickly with minimal frustration. It is the most intuitive and natural feeling, since it is similar to swinging a hammer or throwing a stick.
No-spin throwing, in contrast, requires you to refrain from this natural tendency that you have spent your whole life acquiring. It can be tremendously frustrating and unrewarding. On the other hand, once you get the hang of it, no-spin throwing is extremely satisfying and empowering.
To start no-spin throwing, it is best to keep it simple and not worry about the sub-technique (Russian, Thorn, or Mumyou-Ryu). It is wise, on the other hand, to understand the fundamental principles of the sub-techniques, and to use the principles as a rough guide for appropriate knife length. See
http://imgur.com/CGJig8A For example, it would be unwise to attempt Thorn-style throwing with a 8-inch knife, and it would be unwise to attempt traditional Japanese shurikenjutsu with a 13" bowie.