I have a Bissell ProHeat 2x; I really like it. Friend of mine has one too, and it's lasted her almost 3 years so far with regular use every weekend (she has 4 kids and 2 dogs, lol). It has the little upholstery attachment which comes in handy for really scrubbing hard-to-get stains on the floor as well.
I think I mentioned this to you before on the forum, but I used to live in a rental house with white carpets too. Tana had a couple of bouts of diarrhea; Chloe's housebreaking wasn't perfected when I first adopted her, so we had our mishaps while we lived there.
But that carpet cleaner pretty much did the job. Somebody gave me the idea to use a bit of Oxy-Clean in the *clean water* tank on the carpet cleaner. That way, it mixes with the cleaning solution & has a brightening effect. For a really stubborn stain, I made a "paste" of Oxy-Clean and let it soak on the spot for a while, then went over it a few times using the upholstery attachment on the cleaner.
I can honestly tell you those carpets were whiter AFTER we moved out, than they were when we moved in. Seriously- there was some kind of stain- kind of reddish, probably juice, on the living room floor when we rented the place. I never tried to get at it since my throw rugs & furniture covered it. But when we moved out and I did a final deep cleaning, I tackled that stain as well & it came completely out. LOL.
I will say that it's probably not a good idea to put the Oxy-Clean in the clean water tank every single time....it does cause a little bit of buildup and over time, would probably cause the whole water delivery & heating system to mess up. But once in a while for an overall touch up or to tackle a particularly bad stain, it won't hurt anything.
As for the cleaning solutions- honestly, I never noticed a bit of difference between the expensive "Pet formulated" stuff made by Bissell or Hoover, and the cheap Walmart ones. Bissell makes one that smells like lavender, and it really does make the house smell nice for a while; the odor of the Walmart brand is more boring & doesn't last as long. But as far as cleaning, I never noticed a difference.