Originally Posted by indyfan
any reason why you want the polymer HK pistols?
If I had to buy an HK, I would go with the USP9 or USP45 (Tactical versions if available)
It's a little more pricey than their polymer pistols, but I haven't heard any real good things about the HK45 and P30.
and anyway, if do decide to go Polymer, there's only one handgun you should be buying, I won't say it because I don't wanna start a thread war.
but let's just say it rhymes with something you put on your feet.
Just to note that the USP's are also polymer. In fact, virtually all of HK's current pistol line ups are polymer since HK's main target market is military and LE; not a lot of 'em carry heavy steel sidearms anymore.
The HK45, as someone else mentioned, is pretty much a "next gen" USP and the intention was to submit it for the US Military's Joint Combat Pistol program for appraisal and testing towards the next approved standard issue sidearm. Unfortunately the US military scrapped the program before evaluation could begin.
The P30 and its variants are aimed at the police market, hence the "P" designation, but for all intents and purposes it too is based on the USP platform. It was supposed to be an upgrade to the P2000 (which also is engineered around the USP platform) and was originally be to called the P3000.
However, the P30/L/LS do not use the HK recoil reduction spring system like full-sized USP's. Instead, it uses a synthetic (i.e. nylon) recoil buffer.
If you've never held one you have to...wow! The best description I have heard is that it just melts into your hand and, yeah, I gotta agree!
The advantage of going HK45, or P30/L/LS over the USP is that they have the new features of modular grips. In addition, the OP may want to mount a tactical flashlight and the newer HK's use standard picatinny rails whereas the USP's still use HK's proprietary rail system from the early 90's so you either have to use expensive LAM's and lights designed for the USP or buy a picatinny rail adaptor --Why don't they offer USP's with a picatinny rail as standard instead now that we're in the 21st century? Who knows?
To answer the OP's question, the caliber is personal preference. I highly doubt 9mm is a wimpy round for defence as some have made it out to be. I certainly wouldn't want to stand down range with an apple on my head and let anyone shoot at me with a 9mm! Would you?
Not to mention if you're in a civilian self-defence scenario you'll probably going to be shooting at less than 20 feet or less and you better be damn sure what/who you're about to take down.
If you're concerned about 9mm stopping power you can, as others have suggested, load it ready with +P JHP's (if legal in your locality), instead of FMJ's. Not to mention JHP's might better avoid dangerous ricochets and strays from hitting innocent bystanders while you defend yourself, loved ones, or your property from a criminal.
Also as many others have pointed you're more prone to practice shooting more the less it costs you and 9mm is A LOT cheaper to shoot with than 45 ACP.
However, if you want the best of both worlds (larger caliber and increased capacity) then consider the P30/L/LS in .40 S&W.