I've just apgraded to my 3rd and most powerfull porty so can add a couple of things. My first machine was a a prochem 100psi single vac, second was a ninja 135psi twin vac and the latest is eclipse 500psi triple vac.
First thing I'd say about having the 500psi machine is its great to have extra pressure available when needed! I use it around 150/200psi most of the time but have cranked it right up on several occasions for really trashed and commercial carpets.
The difference it makes is the speed flushing out the dirt, normally a single pass perhaps 2 on the worst bits when with a lower powered machine it might take 4,5 or even more! Follow up with a dry pass or 2 and I've recovered most if not all of the extra water put down.
For example on speed difference I do a really dirty wine bar carpet every 2 or 3 months, did it for the first time with eclipse Sunday night and I was in and out in 1 hour and 45 minutes. My best time with other machines was 2 1/2 hours!
And this time it was dirtier than usual because it hadn't been done for nearly 6 months because they were going to change it but decided otherwise.
In my opinion: You can do a good job with a 100/135psi machine however having a bit more power in reserve is very usefull and will definately speed up your work time on certain jobs.
Kind regards,
Jason.