Both great scopes. It depends how you are going to use it. Since the reticle is set for the highest magnification, depending on the recoil of your rifle, the 15x may help you spot your own long distance shots better than the 22x. The 22x would be great for a dedicated Prairie dog scope.
As far as the 50mm vs 56mm, you have it right. The lower scope is nice, since the lower a scope is to the bore of a rifle, the less canting of the rifle will affect you (although you should strive to keep it level). Also, its typically easier to get a good cheek weld with a lower scope, although depending on your stock and any adjustment you have there, that might be a mute point. The 56mm will theoretically get you more light, but the 50's that I have are already great in that regard.
Choosing between those scopes, you can't make a 'bad' decision. You are gauranteed to get a great scope.
(btw: mine are both 3.5x15x50's with R1 reticle, so I'm probably biased).
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