I agree with Leo45 and batty. It really depends on the individual. The subject of which type of background is best is subjective to say the least. My own experience was as a QFI self improver but I do not think that was the most important factor. It was that I kept my head down and listened to the training captains and flew the sim/aeroplane in the manner they wanted (or at least as best as I could!!). The guys/gals who make it through training are most likely to be the ones who learn the SOPīs inside out, prepare thoroughly beforehand, show a willingness to learn, display sound airmanship etc etc.
The above is only based on my own experience and from what I have seen of the pilots who fail to make the line. Lastly, my previous company used to have a policy of only recruiting those applicants with commercial experience (or ex military) then as an experiment they recruited some CAP 509ers and guess what..... they were able to complete the course. In fact the Chief Trainer said that some of them were as good as guys already on line!
BBK