The HK45C is lighter overall and, for me, has a more ergonomic grip. The USP has better greip traction, but I usually thrown on Hogue slip grips on the Compact and that suits me fine. I think overall if you think the weight difference will be an issue, go with the 45C, but you can't go wrong either way. In addition to the full sized model, the HK45 is sold in compact and tactical versions. The primary difference between the three models is overall length and barrel length, with the tactical. 
Montrala HK45 and HK45C returns to Europe
HK45/HK45C tritium night sights, green/green: 119: 50970172: 642230255326: HK45/HK45C/USP45/USP45C Tactical thread cap, M16x1 LH: 19: Learn More . KEY FEATURES. TECHNOLOGY. Cold Hammer Forged Polygonal Bore. Learn More. During the 1960s, Heckler & Koch perfected the famous polygonal profile barrel technology. Conventional lands-and-grooves. HK45 vs HK45C California handguns Home: My iTrader: Join the NRA: Donate to CGSSA: Sponsors: CGN Google Search: CA Semiauto Ban(AW)ID Flowchart. Ergonomically HK 45 was a bit better fit with the small backstop for my smallish hands (opposite your problem). _____ ". when a man has shot an elephant his life is full"- John Alfred Jordan.
Heckler & Koch HK45 For Sale Hk Usa Hk45 Compact V1 45Acp 3.94" 8Rd. 2 more deals from kygunco.com 689.99 View Deal Heckler & Koch Hk45C guns.com 699.99 View Deal Tabletop Comparison Customary Metric Drag/drop and reorder the handguns to compare on top of eachother Handguns FN FNX-45 Opacity View Heckler & Koch HK45 Opacity View We directly compare the venerable 1911 pistol to the more modern HK45c in features, handling, accuracy, and more.https://www.facebook.com/TritacShooting/Inst. 
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The Arsenal Handguns HK45 vs HK45c g8tr May 31, 2013 The #1 community for Gun Owners of the Northeast Member Benefits: No ad networks! Discuss all aspects of firearm ownership Discuss anti-gun legislation Buy, sell, and trade in the classified section Chat with Local gun shops, ranges, trainers & other businesses For size, the HK45 is a bigger gun, longer, taller and about a half inch taller even. Here is the P30 laid over the HK45 One difference between the guns is that the P30 has removable grip panels, and it comes with two other thicknesses, whereas the HK45 just comes with one extra backstrap.
The 45C system is specified as the "modified Browning linkless recoil operating system" to help mitigate recoil. The USP Compact uses a "specially designed captive recoil spring assembly with. The 45C is definitely a better carry weapon. I thought it would replace my P2000SK as a summer carry gun, but it hasn't. If CCW of this purchase is the primary factor then go with the compact. I use different guns in summer vs. winter when the grip will be worn next to skin and the P30/HK45 grip is rough. 
P30 P30L HK45 HK45c PIC THREAD Page 2
HK45 / HK45C Extended Barrel, threaded. HK45 models can be fitted with threaded barrels for use with muzzle mounted sound suppressors. The HK cold hammer forged polygonal barrel is engineered for increased. accuracy, improve service life and user safety in case of obstructed bore occurrences. Heckler & Koch also manufactures the HK45 Compact (HK45C). The HK45C features the same improvements as the full-size HK45, but has a shorter, more conventional straight grip similar to Heckler & Koch's P2000. The grip can be customized with interchangeable backstraps.
Hk Usa Hk45 Compact V1 45Acp 3.94" 8Rd. 2 more deals from kygunco.com 689.99 View Deal Heckler & Koch Hk45C guns.com 699.99 View Deal Tabletop Comparison Customary Metric Drag/drop and reorder the handguns to compare on top of eachother Handguns Heckler & Koch USP 45 Opacity View Heckler & Koch HK45 Opacity View From what I've read they are pretty much the same except for the grip. if you are thinking about carrying it, go with the 45C if you are going to use it for targets and home defense I think the regular 45 would be better. 
HK45c and G30S
The HK45 was supposed to be a more refined and updated version of the USP. It has superior ergonomics (i.e. thinner grip, backstrap replacements) and a 1913. hk45c. + newer design. standard rail. + ambidextrous slide release. + slide is more sculpted and thinner. + probably about as compact as you are going to get for 8 rounds of 45. + adjustable grip sizes. - rail inexplicably to short to work with a surefire x300 light. - grip textured so minimally it is almost slick.
