There's no end of advice about this topic. Most of it is reasonable, too. My own experience was to go from a .22 first gun to a .45 as a second gun. The big reason for that progression was that I had started Bullseye competition and the basic guns needed were those two. Nines had not yet entered the picture (and still are only minor players, except in Leg matches).
The reasons for a 9mm are compelling - the ammo is readily available and relatively inexpensive. Also, the 9mm Luger cartridge is an easy cartridge to load for and there is a good selection of bullets and data.
The reasons going with a .45 are also good. Ditto the facts that I just mentioned about the 9mm. There are even more bullets and more data available for reloading the .45.
I have been told by folks that I respect that the .45 is an inherently more accurate cartridge. My experience has borne that out BUT I wonder if it is not a function of the fact that I have shot the .45 more often and own more sophisticated pistols for that cartridge. There is, however, no denying the fact that if you go to a Bullseye match and walk down the line looking at who's shooting what, you'll have a hard time finding a 9mm.
What are your intentions for using the gun? Do you intend to reload? Do you want to carry (9's, size being equal, tend to have greater magazine capacity)? How demanding are you about accuracy? (and at what distances?)
One of the things that I like most about most about shooting a .45 is the gun. I use 1911s. There are literally thousands of parts and aids available to the shooter for that gun. Want to shoot a lighter load - something that won't cycle a "regular" gun? Change the recoil spring (30 secs.).
That being said, your choice, perhaps, of the new XD simplifies your use. You may never have any desire to do anything to it other than shoot it. A buddy of mine just bought one and is very happy with it.
About recoil - you get used to recoil. If, however, you are going to shoot a lot of factory ball ammo, you may find, as was mentioned, that the 9mm is kinder to your joints.
We have a shooter at our range who is just beginning to participate in match shooting. He owns one .45 - a HK tactical pistol. The gun beats him up when he has to shoot offhand. I can see it happen as the match goes on. The gun as it is won't function with target loads and lighter springs and such are hard to find. Would he be better off with a nine? Maybe it would'nt hurt so much (remember, shooting with one hand is harder on the joints.).
What's he going to do? Buy a 1911.
You'll have fun no matter what.
Pete