Here's what I think I know:
"Light gathering" is influenced by two things: Exit pupil (the objective lense size in MM divided by magnification) and lense quality and coatings. My understanding is that the human eye can only take in about the equivalent of an "8" exit pupil, which qualifies all of the scopes you mention. So theoretically, the 32mm will give you all the brightness you can use. As eyes age, they lose the ability to take in light, so someone my age might benefit maximally from a "7" or less. Lenses and coatings can be very expensive, and often are the biggest driver in cost of a scope. I have all three of the scopes that you mention. I cannot honestly tell you which is the brightest as I find them all satisfactory, and I have never done a real side by side comparison in dim light.
The scope I have used the most is the 4X40 Buckmaster and probably slew a couple dozen deer with it on my .308 rifle. I replaced it with a Bushnell 4200 for the Rainguard feature, not for any dissatisfaction with the Buckmaster. I also have a 4X40 Monarch that I mounted on my Savage muzzleloader. I never compared the Monarch to the Buckmaster since I have always found the Buckmaster to be perfectly adequate at all legal shooting times. The first morning I pulled that Monarch up at legal shooting time it was still pretty dark and I was surprised at how bright it was. But I never looked at the side by side in the dim light. I'm gonna guess that when I do, the difference in lenses and coatings will show up and the MOnarch will be brighter. The exit pupil is the same on both = "10".
As I said, I have always been perfectly satisfied with the Buckmaster 4 X 40. I do have another thing I like about it. Last fall when I put it on my 20 gauge NEF Ultra, I had to zero it in with those expensive (like $13-$14 or so per box for 5 slugs)slugs. I shot a group and adjusted the windage and elevation to dial it in. The POI move exactly to where I had dialed - dead nuts. I was impressed.
I don't think you can go wrong with any of these scopes. If I had to pick one based on your foremost needs, I have the most experience with the Buckmaster. Most people would agree that they have really good optics for the price.
If you live anywhere near me (NW Illinois) I'll let you look at all three. Or I'll get up early enough and see what I think in a side by side comparison. My opinion will be worth what it costs.
Just one other thought - every once in a while Natchez fas a sale on Nikon factory refurbs. I believe I have seen the refurbed 4X40 Buckmaster for about $100. I would think that it would be good as new - heckuva savings.
