It is actually useful to have both FAA and JAA ratings. There is still a number of corporate aircraft based in the UK on the N register (I understand that D of T is trying to clamp down on this), therefore if you have both ratings it may open up some other options on the job front. You can of course do the FAA conversions afterwards, but it is more cost effective to FAA first and then convert to JAA. Given my time again I would have done FAA ratings first, rather than doing JAA approved training in the US.