Each have their merits, RoSPA I believe retest every three years and the IAM, of which I am a member and ex Senior Observer, has been changing over the last ten years, introducing more industry acceptable standards with "recognised" qualifications etc....
These include a "First" and a "Masters" programme.... I am not involved anymore so would suggest looking at both and maybe asking if they do an assessment ride to let you know where they think you are at.... can be quite helpful... and eye opening.
I would always encourage anyone, regardless of how long they have been riding, to invest in personal advanced rider training, possibly the best accessory spend you could make.
Of course once having completed the course and hopefully qualified as an "Advanced RIder" many riders fail to continue to apply what they have learned, usually resulting in becoming a statistic.....
That and it matters not how many pieces of paper you have or how qualified you may become, someday, somewhere, some idiot will still get you and that idiot may even be oneself.