You will get no credit for your FAA IR when training for a JAA IR. Therefore it is cheaper to complete a JAA IR only.
True at the moment, although if you become a competent instrument pilot under the FAA rules then you are sure to pass your JAA IR 'first go' and when you look how expensive it is per hour to do the JAA IR you may save some money. Besides in the future it may all change, I did hear rumours that an ICAO IR holder may be offered credit in the future.
Also, I'm sure you're aware, but the 50 hrs PIC X/C required by the FAA must be over 50nm distant from take off to landing at another airport. And remember the FAA IR can be used in an N reg plane anywhere, so legally you can fly IFR in the UK with FAA IR in a US registered aircraft........
Just another thing, it depends what you want to do with your IR. If you want to fly as a PPL then the FAA IR will give you the IMC rating on your JAA licence for free. You can also fly IFR in the states and in an N reg as I've already mentioned. The JAA IR is not really an option for a 'private' pilot as it costs so much to maintain.
If you have aspirations to become an airline pilot, you may want to forget JAA for the time being and train in the US. Get your FAA CPL IR and CFI / CFII ratings and work as an instructor for a few years (getting paid of course), get 1500 hrs and the FAA ATP. Once you have the FAA ATP, then there is NO requirement to do the FULL JAA IR course. To convert to JAA ATPL you need to do the ATPL writtens, and basically be at a standard good enough to do the JAA IR check flight, and signed off by a FTO as such (which should be no problem with some hours flying in the UK). It'll cost a bit to convert the ATP to ATPL, but at the end of the day you'll be far more employable that someone who only holds around 250 hrs and a 'Frozen' ATPL....
Cheers
EA