I haven't seen anything formal, but, based on an email exchange last year with our design engineer, his best recollection was that the PA18 was tested to have an IP3 of around 10-15 dB while the CPA19 was somewhere between 25 and 30 db.
I'll ask to see if he has anything more definitive.
My general rule of thumb is to reserve the PA18 for rural locations only where there's nothing around. The CPA19, although its a wee bit nosier (perhaps a half a dB or so) can generally be used anywhere a pre-amp is called for. The CPA19 also has a deep notch filter to reduce signals between the high-VHF and the UHF bands, usually military, public service, and general radio communications as shown here:
http://www.antennasdirect.com/clearstreamcpa19gain.html
The CPA19 is also more robust in terms of ESD survivability.