I tried to consider Vilkas, I really did. Since he was the best candidate of the other Circle members to become Harbinger, I wanted to marry him as a "consolidation of power" tactic. Unfortunately, every time I tried to talk with him, he'd either insult someone he "respected" behind their backs or put his foot in his mouth some other way. I'd rather not marry a man who's going to be sweet to my face but talk smack about me when I'm not around. (Plus if the man ever threatened to slit my throat again, I'd have to fus ro dah him across the room on principle, Companion or no.)
Farkas, on the other hand, still showed respect for othe Companions, even those who treated him poorly. He swore to stand at my back against the world at the end of Proving Honor, and was decimated when Skjor and Kodlak died. THIS was the man I wanted protecting my children.