So, now I own a full sized M&P 9 that has a mag safety and just purchased an M&P Shield that does not. I want to have both guns match, either with or without, I don't care which. Assuming it needs to be done by S&W or a gunsmith, have you any advice as to which is easier/cheaper? Remove it from the FS or add one to the Shield?

I should add that the shield manual of arms says they come both ways though I have yet to hear of one that has shipped with a mag safety.