I would go with the Rug Doctor, but look into an Oreck Orbiter as a companion machine. Before I deep clean any carpet, I bonnet clean the carpet to loosen soil and remove stains. Then I simply rinse with my non-SpinScrub Steamvac. The results are amazing.  I wasted too many hours working with just an extractor, when I with the Orbiter I can clean in less time for a better job. You will not regret it. Low pile carpet like berber and commercial grade tend to be best suited for a bonnet cleaning combo from what I hear. 

The Rug Doctor Mighty Pro is exactly the same machine they rent out, and I often see those machines run perfectly with hundreds of hours on them. The performance at the hose is excellent as well. If you haven't already, rent one for a few hours and see how you like it.