Really there are 3 basic categories:
Agencies
Brothels
Independents
Most 'independent' AW listings are really either brothels in disguise or girls working in a brothel. There are are few true independents, but at least they are probably more likely to show up on UKP because they tend to be around longer with the same profile.
With that in mind, my experience would be split up like this:
Agencies:
Expensive, but more consistently reliable, better service, younger and 'fresher'
True independents:
Good, but sometimes older and a bit 'fake'
Brothels (including fake independents):
Very variable. Some of my best punts, but overall more bad, going on hideous, than good.
I think agencies and true independents have the opportunity to look after their reputation. Brothels, being illegal, must find it difficult to put down roots and build up a good reputation. It would make sense for them to get the cheapest (and hence worst) girls and then rely on bait and switch to entice us one time suckers in.