Parkbreak, by the sounds of it you're doing instructing as a means to an end with the intent to move onto charter when you can. Now before someone shoots this down, I did it as a means to an end but I did actually want to instruct and do love teaching. I'm not going to assume that you are anything other because I'm sure you've dealt with crappy instructors yourself.
If this is the case, then you need to factor this into your plan. You need to look beyond the hourly rate.
Some better paying, bigger schools look great but they can take more time to get on your feet and get decent hours. They will also limit your progression into charter. Smaller/cheaper schools will often see faster progression and often branch into more charter ops.
Ask the instructors at each school how long its taken them to get on their feet and the number of new students walking through the door. Also ask them a rough timeframe for progression onto charter, multi IFR training and finally greener pastures. The last thing to ask is a rough idea of their hours in the first and 2nd years.
If you're earning a great hourly rate (award) but doing stuff all hours with very slow progression you're in a far worse position than the guy raking in the hours. Remember that hours mean progression. The faster you build your hours the faster you move onto grade 2, 1 and multi IFR. Yeah, ok, its at a lesser pay rate to your peers but what about the guy you did your instructor rating with who's still a grade 3?
The final caution with my advice though; Good hours with a "dodgy" operator are always subject to high scrutiny. So ask around for what people think/say. Unfortunatly rumours are often worth more than facts in this industry.
N.B. I'm not saying do it for nothing or to offer yourself for less but if you have a choice dont just look at the hourly rate.