As the regulars on here will know, I am a BIG fan of the FAA system - for a multitude of reasons. (I have CAA and FAA licences).
From what you've written, your best option is almost certainly to renew your CAA licence. (Unless as one of the other contributors to this thread pointed out, you plan on flying mainly when on holiday in the USA.)
There will be more effort involved in getting a stand alone FAA PPL because you have to pass a written test (admittedly not a very difficult one!) and because the FAA flight test includes things which you may not have had to do before and which whilst not hugely difficult, will require some practice.
To renew your CAA licence will involve paying the CAA some money for a new five year licence (daylight robbery I'm afraid) and passing a skills test with an examiner. All told, this will be easier and quite probably cheaper.