The Mark IV wins hands down. Aside from it being very accurate right out of the box, it has one of the largest selection of aftermarket parts available. You can't go wrong with a Ruger Mk of any kind.
Wife has a Ruger 22/45 light. I have a browning Buckmark.
I prefer the buck mark.
Ruger is just as much of a PIA I feel to take apart and from experiance, needs to be taken apart and cleaned more often.
There is a spring in the ruger that can be tricky so I watch the video every time since its the wife's and she usually shoots it.
Both are easy to put a red dot/reflex sight.
I have run both from a rest to look at accuracy and used various ammo. Ruger was more accurate from a rest.
Ruger as mentioned has lots of upgrades like triggers. I have run both stock though.
Weight on my buckmark is heavier and one handed, I shoot the buckmark better cause it moves less with my trigger pull. I"m sure with a trigger upgrade the Ruger would shoot better than it does.
I did a test on both with minimag 36g copper coated. Ruger lasted about 500 rounds before it needed cleaning (FTE/FTF). Buckmark ran well over 1000 rounds and I did not even have a hiccup. I cleaned the breech and bolt face w/o dissassembling it at 1100 rounds and ran another 500 and even though it did not fail, I dissassembled and cleaned it anyway.
I feel the mag on the buckmark has a better feel in inserting it. Her ruger, I have to double check its in all the way or it drops after the first or 2nd round is fired.
Ruger was a threaded barrel so it was suppressor ready. The buckmark I got was not.
Ruger needed a mod to the bolt release button. Buckmark was GTG out of the box.
I have not done a trigger mod as mentioned but strait out of the box, Buckmark was much better IMO. If you get the 22/45, I feel be prepared to do mods.
I have had both since 2013
They are honestly both accurate as the shooter and you will need to be a VERY experienced shooter to know the difference or otherwise, its mostly the fit and weight that would make the difference. My wife has small hands and much prefers the light gun. She has shot my Buckmark and did not like the weight. As mentioned, me personally, I shoot the buckmark better one handed, but 2 handed they are pretty much the same.
Wife's cousin was in town and friday we took her to shoot the 22/45, by the time we were done, she was hitting 12" steel @ 25 yards 6/10 shots.
You can't go wrong either one you choose, but if you want to do mods, check out what mods you want to do. I just hear (as already mentioned) a Ruger has more out there for mods.
I appreciate the insight into the Buckmark. I'm guessing you have the mark 3 Ruger though?