The AFI (oops FI(R)) course is a major undertaking. For me it was the most instructive "rating" I ever did. It taught me a lot about flying as well as how to instruct and I learnt about myself as well. It is really much more than just a simple rating too - it opens up a complete new dimension to flying, not to mention an opportunity to fly without cost and even earn money from it!!
Who you choose is key - others have mentioned some of the more obvious names. I would recommend most of those explicitly or implicitly ID'd by previous threads, except perhaps one of them. Even in a rumour network there comes a point where you have to make your own personal choice, because the most successful choice depends upon matching personalities.
Before you decide you should go and talk at length to your potential instructor, and preferably fly with him too. Everyone gets on better with some people than others. (I am not sure but I do not think that there are any female instructor instructors yet)
The course is/should be very intense. I would strongly recommend that you try and do your course with another student instructor. Working as a pair with the instructor gives more opportunities to do "mutual" briefing and flying. It can also avoid some of the artificiality of pretending to "teach" a flying guru to fly.
Finally, you are more likely to get that elusive opportunity to instruct at a school that has solid links with the person who taught you to instruct. He will know you by the end of the course quite well, and be able to advise potenial employers how you are likely to turn out as an instructor.
Good luck, and above all ENJOY IT.