I don't know, early issues OA tend to bring a premium. Plus, "better artist" is a totally subjective thing.
Chew, for example. Some people love the art. Some people can't stand it. It's just a matter of personal preference.
For the record, I like both artists work, but do tend to prefer Adlard on TWD...I like Moore's Ghost Rider, though.
Yes, the pricing for art is vastly different for the two. You might say it is the scarcity Moore's work compared to Adlard's. You might say it is the "early issue preimuim". However, check the pricing on commissions. People are willing to pay a lot more for a Moore commission than one from Charlie.
Personally, I understand people saying they like Adlard's stuff on TWD. It sets a sort of mood. Also, Adlard can draw well. I have a few pages of his that I like. However, I see a lot of his pages with little detail and that just seem rushed. It is disappointing. maybe because I like OA.