Hitachi is the OE manufacturer. Don't know much about Dorman, but I am sure there are other manufacturers (I think Dunlop is another) offering bolt-on replacements for the RRS' Hitachi. If you can wing it though, look for the AMK compressor replacement kit (comes with new bracket, acoustic box, relay and air line fittings), it is the new OEM for later model years RRS (2012+) and a better design overall than the Hitachi, worth the extra expense IMHO. That's what the dealer would install as replacement for yours as they discontinued the Hitachi model. Besides the higher price, the AMK also requires an update to your EAS module software after installation.
I am replacing it because I got the suspension fault. I took it Land Rover and they ran the diagnostics on it and said it is weak and needs to be replaced as well as the main control block. I wanted to do it myself because they quoted me $2000 for the repairs on both. I would like to do the control block as well but I couldn't find any threads on it.