C150
This may not help you if you are too far down the track, but perhaps it may help others approaching your situation.
When i decided that an an instructor rating was the ideal enhancement to my bare CPl/IR with no employment prospects I considered it this way. I'm a customer with a few thousand dollars to spend on that instructor rating, so I'll spend a few hundred extra on the road finding the right instructor school. I spent a few weeks on the road travelling, asking questions and "interviewing" instructor school's to see where my money would best be spent. I was not looking for the cheapest school rather the one most likely to employ me after I'd spent the money. I took into acount demographics, city/ town size, number of schools servicing, number of curently employed instructors etc, etc, etc. At the end of my search I chose a school that was right for me at the time. My Instructor Rating was completed in six weeks and a full time instructing job was offered two days after completion. Some would say I was lucky, perhaps so. I prefer to think that a little time spent choosing wisely reaps it's rewards.