Decapther is the cheapest but the worst, do not believe anyone that says they don't have problems with them, because if that's true they aren't using them enough.
For one decaptcher.com uses the worst payment gateway on the face of the planet, avantgate, where if you type in avantgate sucks or avangate complaints you'll see a page with hundreds of complaints about how they are racist against americans and all sorts of other things. IN fact I tried paying with a verified by visa credit card that I use everywhere else from a comcast IP address that GEOIP will show is within 5 miles of my registered address an scamgate told me I had to fax them a copy of my drivers license to pay decapther.com $10, yeah right, that isn't happening.
They suck during the daytime, and I can prove that to anyone. During USA daytime hours you'll get the "server overload" message constantly, and when solving captchs wheN I wrote a test script, they only get it right about 60% of the time, if you combine that and go by a 24 hour time period and count the 8 hours a day they give you the server overload message their success rate is under 40%