I'd rather take the full PPL than obtain a "restricted certificate" (which by the way seems to be as simple as knitting fog to obtain since 9/11).
Why? The only real advantage to obtaining "the full PPL" is that it would permit you to retain your FAA privileges in the event that you don't maintain your JAA PPL. The disadvantages are that it will be more complicated to obtain than a "restricted PPL" (piggybacking on your JAA license) - which btw is still quite easy to get, 9/11 or no - and (I think) it will require you to maintain a separate FAA medical category.
The above assumes that you will be content with PPL privileges... if it is your ambition to go on to an FAA CPL or ATPL, then perhaps it would be necessary to first get a "full PPL", although I'm not so sure about that.
MLS-12D
P.S. The article
Goodbye CAA PPL/IR may be of come interest, although possibly slightly outdated (June 2001).