Considering the enormous number of questions posed on this forum and other places, the concepts seem fundamentally flawed (because both are formally and practically unobservable). The calculations themselves (together with the Lorentz Transform) are highly error-prone and the results misleading (the "Mr Tomkinson" phenomenon) and unsatisfying (you can't directly see either except in very specific circumstances). I realize this might come across as flamebait, but as an amateur learner I have not found either concept particularly useful to my studies. They seem most common in pop science "wow look how weird relativity is" presentations, and the fallout is seen in all of the forums that I visit (here, Physics Stack Exchange, reddit/AskScience etc) As an alternative, I would propose approaches that I have seen in a few places, but which are not very common. 1) Formal derivation of the Lorentz Transform, leading swiftly to defnition of the space-time interval and space-time diagrams. 2) Introduction of four-vectors, including four-momentum and four-frequency 3) Extension of the 1+1 spacetime to 2+1 (which is necessary and sufficient for first-person calculations of what we can see/observe/measure) 4) Develop formulas for aberration/Doppler/headlight effects, so students will be able to calculate eg. the shift in apparent positions etc. of stars/galaxies 5) maybe introduce time dilation etc as a historical formality for the interested (or bored!) student I'll leave it here for now, rest assure that I have other practical arguments in reserve if anyone deems this post worthy of comment!