45 caliber
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I don't care what the salesman says your wife will like about a 40cal vs a 45 cal. ~ RECOIL is a 40cal, takes a lot to handle, IMO, non-forgiving, and all the 45 cal's I have shot have far less recoil, and more of a even push, and you both will be able to re-acquire the target faster, and with far more accuracy with a 45 cal.
Go to a range and shoot both.. then you'll know what the difference is in recoil vs the 45 cal.
40cal goes SNAP ^.. 45cal goes < Boom
LS
PS.. if money, ammo cost, is a concern, then you should REALLY look into a 9mm.. That gun will work for both you and the little lady, that I'm sure of.. 115gr FMJ very little recoil, re-acquiring of target for rapid shots is there for less money and more ammo than both the 40cal and the 45cal. And, like the 40cals grip, thinner than the 45's, a 9mm holds, depending where you live, 10, 14 or 17 round magazines vs 8 -10 rounds for most 45's due to both size of the bullets and the wider grip of the 45 to hold mag and ammo, in many of the 45 makes/models.