I would reckon it will be in your best interests long term for job prospects, to convert to JAA if you are planning on living in the UK for any length of time.
I think there is a simpler conversion from FAA to JAA if you have enough multi-pilot time on aircraft of a certain weight, which i believe avoids you having to sit the 14 JAA ATPL exams (which are not nice!). Check LASORS and it should advise you on the criteria, but i know that not everyone has to do a full conversion if they have the right type and amount of time.