What are the main differences between these two handling techniques?
Putting it briefly, Jaakko Turn and Reverse Spin are used for different purposes and in different kind of situations. A Jaakko Turn is useful when you need your dog to do a really tight turn. A Reverse Spin, on the other hand, is used when you need to just make an adjustment to the dog's line, or to eliminate off-course obstacles from his direct view.
There is also a difference in how you prepare the dog for the turn. A slow down cue is always required for a Jaakko Turn, whereas in the Reverse Spin no rhythm change is needed. In the Jaakko Turn the handler is in front of the wing, on the take off side of the obstacle.
You can learn more from the OneMind Dogs handling technique videos which guide you through the teaching stages of each handling technique in great detail. You also get to see what the most common mistakes related to the subject in question are, which makes it easier for you to pay attention to the relevant points.