Well you answered your own question there, if you allready own the M&P 40 fullsize just get the compact. The mags from the FS will work in the compact with the XGRIP sleeve -XGRIP-for-S&W-M&Pc-and-other-guns.
Nah, I don't have an M&P 40...I have a G22. If I already had an M&P, I wouldn't have wasted people's time by asking about getting a compact.
For caliber reciprocity, I'm probably gonna go with the .40
I know alot of people suggest try before you buy, but I believe I am gonna camp outside of FS on sat morning and go straight for the paperwork.
If I can't figure out a way to effectively use/shoot this gun, then there is a problem with me that I'll just have to work on. I understand some guns just "fit" better, and some people shoot certain guns more accurately than others, but I'm not gonna use either of those as reasons not to buy a gun that is as established as the m&p's.
Same reason why I won't hesitate to buy a Sig when I feel like it. Most quality pistols are inherently accurate/reliable, so there is no reason not to own atleast one of each.