Are you planning on getting a USP in .45?? (And like others, I would not consider selling a SIG for a P99, but that's just me....)
If so, keep in mind that the HK USP in .45 is BIG! My shooting buddy has one and it's a great gun, it just doesn't fit my hand as well as the SIG P220. It also is different in it's controls layout.
If you're looking to add a .45, I'd say keep both as you have years of muscle memory handling the SIG P226 and the P220 will be an easy transition.
Regarding recoil, whoever told you that about the P226 recoil was full of it. The P226 is a very soft shooting handgun. The high bbl is a non-issue IMHO.
The P220 is surprisingly easy to control. I thought that the high bbl would cause more muzzle flip than the 1911 that I'm use to shooting. But I was not really able to discern any additional recoil with the P220 over my 1911. I personally like the P220 over any other DA .45 semiauto, but that probably comes from my years of experience with the P226.
If you have to let one go, I'd say ditch the P220 for the following reasons: you've had the P226 longer and presumably know that its a reliable firearm; you have experience shooting it; your wife will be better off with the P226 in a pinch over the P220; and ammo supply favors the 9mm.
I don't particularly care whether my handgun is in 9mm versus .45. If I do my part (and have appropriate ammo in the 9mm), it will be effective. I would only grab my 1911 over a 9mm as I shoot more (and better) with my 1911 than any of my 9mm's.
But I would seriously reconsider where you're going with a potential HK or P99 purchase. The HK is a great gun, but there is a lot to be said for muscle memory between handguns (i.e., if you're going to get a new .45, keep the P220).
Just my $0.02.