imported post
Let's get recoil out of the way first thing. Recoil could be said to fall under three categories: perceived, felt, and actual.
Perceived recoil is the shooter's perception of recoil; his sensitivity to this, which can be a factor of anticipation and perhaps a little anxiety. Training and practice can go a long way to assuage this effect.
Felt recoil is real and primarily involves the shooter/firearm relationship; how the gun sits and feels in the hand. Let's face it, some guns just do not feel good to some people and that is likely to increase the felt recoil. Grips, grip frame shape and angle, bore axis-to-hand relationship, the shooter's hand size vs the size of the gun, etc. All are factors with felt recoil.
Actual recoil. You have little control over this. It is the amount of recoil energy generated by the gun/cartridge combination.
I have never been recoil sensitive, though I certainly have owned and fired guns which were just not comfortable to me at all (a 9mm and a 1911 .45 I once owned come to mind).
Ahh, the battle between the 9mm and the .45ACP. You left one out; the .40S&W. If you learn nothing else from this thread, know this. There is more BS floating around when it comes to guns and caliber arguments than you can imagine. Learn to separate the BS from the truth and you will be past the first phase.
Ignore statements such as, "a .45 will knock a man clear off his feet", or "a 9mm will blow his head off", or "dum-dum bullets (yes, you still occasionally here this) are illegal in most states but use them anyway". Be leery when people talk about "knock down power". And be very suspect of those who make wild assertions and try to appear as though they know all there is to know about guns and ammuntion. Even people who work in gun shops. They can be among the worse ones. In the end, it's mostly just garbage, all garbage.
There have been tremendous advances in defensive handgun ammunition over the last ten years. Superb factory loadings are available in all of the popular calibers. But it still boils down to this. Shot placement. Hitting your assailant in places where the bullet will do the most good - for you. This means penetration and effectiveness, and your ability to deliver rounds to target.
The 9mm, .40S&W, and the .45ACP are all excellent choices in their best loadings so do your homework and learn as much as you can. As several have mentioned, do yourself a big favor and fire all three. Also spend a little time evaluating guns to find the one that best suits your wants and needs. If you like the XD, consider the M&P Smith and Wesson and the Glock, too. All of these are fine pistols but what is most important, besides quality, is what feels and works best for you.