H&K is gonna cost a little more, and mags are pretty expensive. Full size USP .45 mags are like $32. I have a USP .45 compact, and mags for me are $58. :((( but oh well. If you wanna know why the USP is more expensive than the SA XD series, go look up the USP on wikipedia and read about the reliability and endurance testing they did on it.
The recoil system, which reduces felt recoil quite a bit, in a different gun was tested with 30,000+ +P rounds, and 6,000 proof rounds without damage or any excessive wear to any major components. Even the weakest components are rated with a life of 20,000 rounds.
My USP is my favorite gun of the 8 that I own. I've been thinking about buying another one, only full size. The gun is extremely easy to maintain, it is comfortable in my hand, comfortable to shoot, surprisingly accurate for such a short barrel (on the compact), and its reliability is a thing of legend. Fact is, if it comes to a life or death situation, there is no gun I'd rather have in my hand than my USP.
In my personal opinion, and I'm sure others will disagree, you just cannot find a better handgun. period. Keep in mind, as with most things, firearms especially, you get what you pay for.
The springfield I have never handled, or fired, and not really researched at all either. (as far as I'm concerned I've got the only polymer frame automatic I need) So I won't comment on it, but I've heard good things. When it comes down to it, its up to you to do your research and make it good. Don't rush off and drop several hundred dollars on something you may not like 100 rounds, or a dozen cleanings down the road.
As for your first question, if you're using a .357 mag for self defense, what grain the bullet is don't make any difference. Its still a hunk of lead flying 1100fps. Not to mention the .357 bullet was literally designed to be a people stopper. I've never shot anybody with my .357 but I'm willing to bet that the grain of the ammo won't make too big a difference.