A very difficult choice. If you look at what AOPA-US does, then AOPA-UK is a poor shadow. But with a tiny percentage of the membership of AOPA-US, that's what we should expect.
I joined AOPA "on principle", on the basis more members should mean more work done. I did the same for the PFA, although I'll never do any of the things PFA exists for.
I also support GAAC - the nearest thing to an umbrella organisation. GAAC has been a real eye-opener: there are something like 60 flying organisations in the UK, representing different interests, and they find it very hard to agree on most things. Their members have different interests, and that means they argue different perspectives.
No surprise, but it does make it hard to present a common voice to the CAA (or worse, to JAA). There are some very old organisations that claim to represent everyone, but they don't.
Send yer money to GAAC - they desperately need it, to fight to protect our airfields against predatory developers, and nimbys who move into a house by an airfield, then start a campaign to close it.
Or join A4A - <a href="http://www.airfields.org.uk" target="_blank">www.airfields.org.uk</a> - all done by e-mail and enthusiasm.