I can pretty much guarantee that 95% of the people on this board will agree that it is a good idea that you get your instructor certificate (at least the initial). Where you get them is up to you, just make sure you consider ALL options and make the decision that is best for you. I don't have any personal experience with either FSI or ATP. But I do have a few close friends that have gone to either one of them. FSI is an excellent school from everything I hear, but as bobbysamd pointed out, they are VERY expensive! My friends that have gone to ATP, in Chicago have generally had good experiences. One thing that they all had to say is that they were trained to pass the DE's checkride. ATP (at least the one in Chicago) has used the same DE's for their checkrides over and over again, year after year. It is
MY opinion that they know the DE's checkride and review these areas only, and rarely anything else, which is why they can do all three in such a short amount of time. You will essentially be using rote memorization in order to pass the ride. This can really bite you in the a$$ later on! I used to work at American Flyers, DPA and we had an Instructor Academy that cost around $2500 for the CFI and CFII. This did not include your MEI, and took about 30-35 days. People on this board have had varying experiences with American Flyers, which in my opinion was dependant on which location they attended, but I would absolutely give a thumbs up to the Instructor Academy at the DPA school in Chicago. You can do a search on this board for more info. about them. Also consider your local FBO. In my opinion, it does not take a big school to make a good CFI. It takes a good, knowledgeable, understanding, and enthusiastic instructor, (that can be found at any of the "big" schools, but also down the road at your local FBO) and a dedicated, hard working student. Your instructor certificate is probably the most challenging certificate you will earn (key word being "earn"). Do some homework, then dig in!!!
As far as the Twin Otter gig goes, you are either extremely lucky if it pans out, or you will have been suckered. Not many insurance companies like having pilots with "low time" flying their airplanes... especially a multiengine turbine. Just don't count your chickens before their hatched. If it works out for you i'd be extremely jealous... the Twin Otter is an awesome airplane!!!
Best of luck!