Isn't Ahnuld Austrian, rather than German?
if you look at the maps provided with "The Coming of Conan", the land Conan is supposed to be from is actually situated in relation to where Austria/Germany would be. So if you take that into account along with the physical descriptions of the character and how he was portrayed in the stories... a thief, but a thief who was also a war loving, high military IQ, hack & slash, beer guzzling barbarian king... Arnold was a damn good Conan.
The Cimmerians were a proto-Celtic people, not a proto-Germanic one. Their names were typically similar to Irish or other Celtic names (Conan is an Irish name). Of course, the ethnicity argument doesn't work in favor of either actor because neither Schwarzenegger nor Momoa are even a little bit Celtic.
Speaking strictly in terms of musculature, physical agility, attitude, and how he was portrayed in the stories, I think Momoa was a better Conan than Arnold was. I could buy Momoa's Conan as a thief, a reaver, a slayer, with gigantic melancholies and gigantic mirth. IMO, Arnold's Conan came across as a slow-moving simpleton in comparison.
Don't get me wrong, I think Milius' movie is vastly superior to Nispel's in pretty much every way. But just taking into consideration the two interpretations of the character itself, I'll take Momoa's over Arnold's.