I really don't think the FAA oral exam is very hard, I haven't taken the JAA tests but would say it's probably easier than those. And only perhaps 1/4 of it is US regulations. It's not as if a single mistake will fail you, as long as you have a reasonable examiner. Mine was very friendly and it was more of a discussion than anything else. He is mainly wanting to make sure that you have a sensible understanding of the information needed to fly safely, and that if you don't know something, you do know where to go to find it out.
The FAA written exam is multiple choice and basically common sense, I tried the practice test and got a passing mark without having done more than a cursory read through the textbook... I did learn it more thoroughly before taking the real thing and got about 95%.
My "UK conversion", combined with C172 conversion and de-rusting after 3 months of not flying, took 3 hours.
Advantages of FAA PPC: Available at a lot more US flight schools, includes mandatory night training (and grants night priveleges), can fly on an FAA class 3 medical which is not as strict as a class 2, lower minimum no. of hours (though you will probably need more than this).
You can later add an FAA IR for instrument priveleges, allowing you to fly IFR in airways etc in an N-reg aircraft and with similar priveleges to an IMC rating in a G-reg aircraft.
You *CAN* fly a G-reg aircraft internationally on an FAA private ticket. (check the ANO)
No minimum no. of hours flown to keep your rating, but you should be doing plenty anyway for safety.
Disadvantages of FAA PPC: have to return to the US every two years, or find an FAA instructor in Britain, to revalidate your certificate with a "flight review". But if you miss this deadline there is no penalty unlike the JAA licence where you must take a much more comprehensive test if you don't revalidate within 24 months.
Similar restrictions to get a tailwheel signoff.
Can't attach UK IMC rating, but you can get an FAA IR for similar money.
In the end, for a private pilot with no immediate commercial intentions, the decision of FAA/JAA PPL is not as critical as whether to learn in the states or the UK. IMHO - I would recommend FAA, in the states.