Here is good information on shell differences:
http://www.ww2f.com/topic/19890-soviet-vs-us-vs-german-artillery/page-2#entry252451
US had better ordnance.
Here is stuff on organization:
http://www.ww2f.com/topic/19890-soviet-vs-us-vs-german-artillery/page-2#entry259046
Here is something I posted on the forums a while ago:
"The US needs to have a LOT of artillery options. The US were true masters of artillery usage (with the one caveat being that good maps needed to be available). They had easy access to excellent guns, excellent ammunition, precalibrated "tapes" that prefigured most of the necessary equations, and often gave the artillery units a lot of autonomy to jump in on any attacks if they had the tubes and ammunition available.
The different artillery systems in the war:
The Wehrmacht could bring localized fire fairly quickly and accurately but could not mass artillery strikes and were not the fastest to react as they had to calculate accurate fire each time.
The Brits could bring down strikes faster but not quite as accurately and their organization could also manage larger concentrations when needed.
The Russians could not respond quickly at all and not so accurately but they were able to mass truly spectacular volumes given time.
The US was almost as fast to respond as the Brits, often more accurately (so long as good maps were available) than the Germans, and could do so frequently in great concentrations. "Time-On-Target" would be an order given to artillery units of strike at a given time and place given to units. Any other unit in range hearing the orders and had the spare ammunition was free to participate. And using their pre-figured tapes they knew when they had to fire so their caliber guns would arrive at the same moment as all the others."
Germans did not prioritize artillery.