I've led tastings with both the FlavorActiV kit and Siebel kit. Both are good, although Siebel's is easier to work with (liquid samples, instead of pellets you have to dissolve) and more extensive. Siebel's kit also has a couple combination samples -- one to simulate "hefeweizen" (clove phenols and banana esters) and one to simulate an old beer (in bad way -- think of a keg that has gone 'round the bend or beer from lines that aren't cleaned nearly enough).
In both cases, you get many samples that don't really apply to homebrewers, but the are still interesting (or even a little funny, in a gross way). In both cases, I also found that you can't just go straight with the directions on adding the special sauce to the beer. Sometimes, it's not enough to really pick it up; sometimes, it's so powerful that it's not authentic. Just be prepared to play around with the concentrations a little. In any case, either is worth doing, but I don't think you need both.
Also, if you teach (or are a part of) a BJCP exam study course that is tied to a BJCP exam, BJCP will provide one free Siebel kit for the class's use (although you are free to invite others). You have to let the BJCP know and get them some information (like a class syllabus), and I recommend giving them at least a couple months notice (AT LEAST) to get the kit to you. They used to provide the FlavorActiV kit, which is how I got to try it.
TL