I've used both PA and Oddsmonkey. I much, much prefer Oddsmonkey. Their match finder software was originally used by PA, but Oddsmonkey decided to set up their own service, so withdrew the use of their software.
When I decided to give matched betting a second go (I only stopped the first time because of illness, not because it went wrong or anything, don't panic!) having access to decent match finding was my main criteria, so I went with Oddsmonkey.
I think they're cheaper, so that's a consideration for anyone starting off. I found because I didn't have much of a starting float the first time, that PA's monthly fee was eating into too much of my profit. Saying that, I found their videos very helpful as a beginner. I've not looked at any of the guides on Oddsmonkey yet.
I'm £92 up, after a week of doing it, and really only doing it for three days out of the week, for an hour or so in the evening. I lucked out and netted £50 from a Betfair rebate offer they emailed to me as an existing customer, so that's accounted for almost half of it.
All of my float is tied up in the football tomorrow, so I can't do anything more until that all settles.