Oh come on, you aren't seriously trying to plug the IPA over Balpa now, are you? It certainly seems like that.
I am a member of both and feel that I am in a position to make a valid comparison. Balpa is not only much larger and stronger but have the expertise on staff to handle most situations. The IPA (or IPF if you are trying to make a real comparison) is several leagues down the list in terms of expertise, experience and clout.
If yoy want to compare them both along the lines of union or pilot club, then the IPA certainly fits the latter much better than Balpa. I would assess the IPA more of a junior pilots club which helps them feel better about trying to find their first jobs.
Balpa has many study groups that have a real effect on the industry. They are more often than not invited to the many conferences and working groups when new procedures or systems and technology are introduced. They have sway in the industry and operate to a high standard of prosecution.
We often see the impatient but ignorant rants on here from people who have little understanding of how things actually work in the industry and in politics. They expect quick, simple fixes to complex and intricate problems. typical knee jerk reactions but like the old RR Spey egines (an old Trident pilot would know about those) are 90% noise and 10% thrust.
To throw the old adage that most pilots are in Balpa just for the legal insurance protection shows the typical ignorance of most non or anti-Balpa noisemakers and assumes an equally ignorant approach to union or association recognition. Most of us who are in Balpa will have researched the benefits of being in the union and concluded that there is far more to it than the usual IPA rhetoric that their legal cover is on a par with that of Balpa's.
The only thing that the IPA have managed to do is split the brotherhood of pilots and give management a neat wedge to drive into any aspiring pilot groups with a divide and conquer strategy. You onky have to look at the debates on here from Jet2 pilots who are in dire need of proper recognition. Amateurs pining for serious recognition getting shafted left, right and centre by their management through joint workers councils. There are other companies that have unilaterally recognised the IPA as the official union and you only have to look at the lifestyles and remuneration standards in those companies and compare them to the ones where Balpa is recognised as the prime representative union for the pilots. It is like comparing chalk and cheese when you compare the standards.
There is much more but I won't waste any more time. To answer the question, "is Balpa a union or pilot club?" the answer is neither. It is an association of British airline pilots which has been growing steadily for many, many years and has some political lobbying power, working study groups that look out for our interests and enough professional expertise on staff to assist the many Company Councils in their day to day work protecting our lifestyles and renumeration.