In HK you have two lines now:
First goes from Mark 23 Mod 0 to all versions of full size USP. USP Compact belongs to this line, but have different recoil system (this system is carried on on second line). Those are going live as long as some militaries will buy them, no a second longer.
Second line is more modern, usually purpose build handguns:
P2000 - ergonomy upgrade over USP Compact with simplified trigger system, made to won German Police tender against P99. Designated for open and concealed carry
P2000SK - sub-compact version of P2000 made for plainclothes police officers, same internals as P2000 excep dual recoil spring system (like now in Gen4 Glocks). Designated for concealed carry.
P30 - strongly upgraded P2000 made for German Border Guard tender - internals same as P2000, but lot’s of ergonomy improovements and bigger mag capacity. Designated for open carry.
P30L - longer slide/barrel version of P30 made for Norwegian Police tender
P30S/P30LS - added manual safety, avaiable with all triggers versions , but makes most sense with V3 (SA/DA) trigger.
HK45 - egronomy wise based on P30 (or co-developed with), modern .45 made for JSP programme - trigger mechanism similar but not interchangeable with P2000/P30 series (some parts common to USP and HK45c). Designated for open carry.
HK45c - ergonomy wise based on P2000 (lacks some improovements from P30/HK45 like relocated magazine lever) compact .45 made for JSP programme - trigger system similar to HK45 (with some parts common to USP and HK45). Designated for open and concealed carry.
Trigger systems line for P30/P30L/P30S/P30LS are detailed here:
http://www.heckler-koch.de/HKWebText/detailPara/1925/137/4/21/263
CDA acronym is used for trigger system called LEM in US.
Can’t say much on HK45 HK45c trigger line, as those are not avaible here.
Basically HK45/HK45c models overlap with HK USP / HK USP Compact (and Mark 23) line. While P2000 line and P30 lines overaps but P2000 series are more slim line / low profile - better suited for CCW use. P30/P30L is good for CCW as well (I can carry my P30L in AIWB holster easily) but are bigger than P2000 and very grippy grip (sic!) can be hard on bare skin or those damm expensice UnderArmour shirts 
For range use or open carry P30 line beats P2000 line in all arreas.
All mess comes from HKs approach to business. They do not make new model and hope someone will buy it. They look for fat tenders, read user requirements and prepare new model to suit this tender. That is how P2000/P2000SK/P30/P30L were made. HK45 and HK45c were made to submit in JSP, but due to JSP cancellation HK decided to go with them to market anyway - but this is exception that just confirm rule.
My personal choice is P2000SK for CCW (will probably ad P30S V3 or V0 for winter to replace P2000 that I sold) and P30L V0 for range/IDPA/multigun use.