Hi Adamlau, I responded to your other thread briefly, but I will reply here as well in more detail. I would most definitely go with the Bissell (I own one, and can affirm it's excellent cleaning performance), over the Rug Doctor for several reasons.
First of all, the Bissell cleans in both forward and reverse. Every Rug Doctor machine only cleans in reverse passes: hence, the Bissell will yield double the carpet agitation than the Rug Doctor. The forward/reverse motion is allowed by an adaptive, spring loaded suction nozzle that accommodates any carpet type without friction resistance.
Second, Rug Doctors are notorious for leaving carpets over saturated to the point where it would take approximately 24 hours to dry. Read the reviews on Amazon or elsewhere: this is a common complaint. However, the Bissell uses the right amount of solution without over saturating (no, it's not light on the solution like typical household units), and coupled with powerful suction, will yield dry carpet in less than 2 hours. In addition, the Bissell directly vents the exhaust air onto the carpet with each pass. The Rug Doctor does not do this.
Third, I like the fact that the solution tank (1.8 gallon capacity) is removable form the machine, so you can bring it with you to the sink and fill it up that way. With the Rug Doctor, you'd have to repeatedly fill up a pitcher or bucket and fill up the tank that built into the machine without trying to make a mess (this is probably an issue for people who are not as strong), so this is another thing to consider.
Lastly, I'd also consider the possibility of the "vibrating brush" on the Rug Doctor going out (outside the warranty period). Repairing it would be a technician task, because it would not be as simple as replacing a brush roll, changing out a belt, replacing the entire floating brush roll mechanism, which can all be done by the user (and the parts are not that expensive, either). I've rented Rug Doctors before that had a disabled brush, and forced me to return it for a machine swap. Yes, the rigors of rental use are far more stressful to a machine than typical household use, but still lies the possibility. FYI, the retail Bissell BigGreens are the same ones they rent out, except the rental units use black brush roll bristles while the retail units use red bristles.
My recommendation would be to rent the bissell before buying. I rented one recently and was fairly pleased with the results. If you're the type of person who's used to using an Oreck upright or other lightweight, you will likely find the Bissell Big Green carpet cleaner to be difficult to push and pull. The rental machine had several places where the cord was damaged - I don't know if the Chinese made Bissell used a cheap cord. I would recommend cleaning the brush area before using if you do rent a Bissell. The rotating brush produced a small blobs of lint that were thrown onto the carpeting. It did seem to clean better than my Hoover spin scrub.
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