Cost is one factor. Another is the after sales service. I am looking at reliable service - whenever the aircon breakdown, I would expect the company to be easily contactable and when reached (not ding dong or passing the baton to one another), I would expect assistance to be rendered as soon they can to rectify the aircon problem. I think a bigger and a reputable company would not have this kind of problem as they would already have their own team of technician to do the work, and not sub con to other technicians.
Just my 2 cents worth.