I have a Shield .40 and love it. My wife has a Shield 9mm and she can manage it very well. In terms of recoil, I don't notice a huge difference at all (220lbs) but my wife (120lbs) claims there is a difference, but it might be simple perception. Either caliber, the Shield is a great gun. Get what fits your inventory the best. 9mm holds one more round but ammo is scarce right now. I can find 10 boxes of .40 for every one box of 9mm currently.

As for holsters...I have one by HiddenHybridHolsters and love it. Great quality and it can be used as an IWB or OWB so it's a good value.