If it was me, I would choose Quentin.
While I don't think women in those days went in to marriage expecting bliss, I don't believe they came to it expecting misery.
[spoiler]Obviously, Quentin and Samantha weren't happy in their marriage, given Quentin's infidelity, but Samantha's only option was to stay with him. Then once he was presumed to be lost at sea she took Gerard's offer of marriage quite seriously, hoping I think, to find some measure of happiness. Once Quentin returned, her chance for happiness with Gerard was snatched away.
Samantha and Quentin had long since come to an understanding of the sad state of their marriage, and Quentin knowing Samantha couldn't truly be happy with him, gave her the choice between himself or Gerard. She chose Gerard, but I think she should have stayed with Quentin, simply because Gerard only wanted to marry her for the wealth she could provide him.[/spoiler]