Ironically, I am currently using the large one. I say ironically because I bought the MP based on the ability to make the grip smaller due to my small hands. Ater a Burwell trigger job, I find that the large insert allows for a more consistent grip and trigger finger placement.
I have pretty big hands, but what I discovered is that with the large palm swell insert, whenever I pulled the gun from my holster and looked at my grip, the bore axis would be slightly (but noticeably) off to one side. I don't experience this with the medium insert. YMMV.
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