We've used both Coit and Stanley Steemer for carpet cleaning at different times. They seem about the same in most ways. In both cases the cleaning was adequate but not much attention was paid to stains or problem-areas.
The cleaners arrived in a big yellow van and power-shampood everything. They moved furniture as needed, and "pre-spotted" areas that were pointed out as having stains, but the cleaner also stated that the cleaning would get everything out without that step - which turned out not to be the case.
Stanley Steemer had less of a hard-sell for extra services than the other company, but for all practical purposes the two services seemed indistinguishable.