Sanders or Clinton: Who would be more effective with a Republican Congress?

I am a firm Sanders supporter, having donated a small amount to his campaign. One of my workmates is a firm Clinton supporter and we have had several spirited and civil discussions of the respective candidates' merits.

His principal reason for supporting Clinton is that he thinks she will be able to work more effectively with a Republican Congress. He makes a compelling case, so I am curious to see what folks here think of this issue.

Who would work better with a Republican Congress?
Does such a concern matter in deciding whom to support?
Should it matter?
Why or why not?