Dick Sargent (the second "Darrin") had been first choice for the role of Darrin but had to decline due to other commitments. When Dick York (the first "Darrin") left the show due to illness, Sargent was again first choice. As for which actor Montgomery preferred, she made very little comment on the topic and was diplomatic about it. She said they were both very good.
She did not keep in touch with York whereas she was friends with Sargent, and had been so for some time before Bewitched. There are rumors about friction between Montgomery and York, but there's no real substantiation and neither of them commented publicly on it; in fact Montgomery has said very little in total about Dick York. There was some hostility from Agnes Moorehead towards Sargent regarding the decision to change cast, but Sargent and Moorhead ended up as friends, according to Sargent.
Rumours aside, Montogomery's out-of-work friendship with Sargent was the opposite to her complete non-communication with York. We are left to speculate on what they may or may not signify.
In 1992 Dick and Elizabeth Montgomery were grand marshals of the Los Angeles Gay Pride Parade. She stated, "In or out of the closet, I love him. He's a super guy and a good friend. I'm happy for him and proud of him." Quote source: http://bewitchvic.tripod.com/sarge96.html