I have both the XD40 and USP40 decock only.
Both feel good in the hand, the angle agrees with me on both of them. The checkering on the plastic is a bit sharper feeling on the USP.
I prefer the mag release on the USP, it is faster for me to reach as I do not have to shift my grip to hit it. To eject a fully loaded mag in the XD takes a ton of pressure. The XD mag release is more a standard design, though both are ambidextrous. Some people don't like the paddle type release of the USP(same on the P99).
The USP feels a bit blockier in the slide, though both are more blockish than most pistols anyways.
The USP has a polygonal grooved barrel, so no-no on lead. Though my USP is one of the first that came out, so it has a standard land and groove barrel. Neither have a history of kabooms, but if you hotrod the .40 S&W, nothing will be safe.
The XD trigger feels a bit squishier than the SA trigger on the USP. Maybe it will be better after a couple 100 more rounds. The trigger on the USP feels wider too, which is helpful for the first full double action shot. I don't have a problem firing quickly with either one. The reset feels about the same on both.
There is no safety on my USP, just decock. So no confusing up for safe, like a 1911, and down for fire and more down for decock. Though there are tons of options for the safety type (decock only, safe and off safe only, D/A only). And now, you have the option for the LEM type trigger on the USP which is more like a SA only type function (like a Para Ordnance LDA the internal hammer is already cocked, so though you have to pull the trigger a full Double Action distance, the weight to pull it back is much much lighter, more like a Single Action pull). Though I am not sure if there is a second strike ability (hammer falls, no boom, pull trigger again?) in an emergency though, loading the next round would probably be better anyways. The XD is simple in that there are just the normal slide release and mag release. With the added striker cocked indicator (kinda like a hammer is cocked, you can see it) and loaded chamber indicator (the new USPs, the extractor functions as a loaded chamber indicator).
Accuracy, most magazine reviews give the edge to the USP. The XD is combat accurate, the USP can easily be made match accurate on top of its already great accuracy. Though you do pay for it.
Recoil, the double spring recoil design on the USP40 works wonders. It is the only .40 my wife will shoot, it works so well you can feel it slow down the slide. The XD is a bit snappier, but not painfull, though there is enough of a difference so my wife doesn't want to fire it much. She can fire the USP all day, and has, as I have stood there and loaded the mags for her.
Price, used USPs are usually pretty reasonable around here, I would never buy one new. My piece cost $469 with night sights and nickel (not stainless) slide. My XD40 Stainless was $450 like new. Mags for the USP are pricey though, cheapest I have found for 10 rounders is $29. Hopefully high caps will drop in price after the ban, instead of $69 and up. The XD has the bonus of being able to use Beretta mags with a simple modification. Though I don't know about the 40 caliber one. Putting a 9mm Beretta 92 mag in the XD40 bent a feed lip because of the ejector hitting it.
Both have been reliable but I haven't fired the XD much yet. I have put about 1000 rounds through my USP but only about 200 through the XD. Full metal jacket and lead semi wadcutters.
Looks, USP, sorry, I just like it a ton more in the looks department! Though the stainless XD doesn't look like it was hit with the ugly stick like the OD green XD.
Good luck, try and fire both, but both are fine in the end I think.