Interesting first post, but I'll entertain it.
I haven't visited the agencies you mentioned, and generally don't use agencies. Price is one factor but there are many more. The pictures are usually heavily photo shopped, you have no feedback of the WG you are seeing, services can be vague and they are generally aimed at the inexperienced punter with more money than sense.
Adultwork is one of several platforms to find Independent WG's where you can see feedback and link their profiles back to UKP to find reviews by fellow punters, which is a great benefit in not finding a lemon. I wouldn't rule out ALL agencies but many of them are rip off merchants and will probably leave you disappointed unless they have been reviewed positively here.
If I fancy a late night last minute punt, I may use an agency, or if one WG particularly catches my eye, i'll go for it but generally I have a better time with your average indy WG, with selfie pictures, rather than a stuck up beauty.
