Who to pick for 3rd letter? (Computer Science)

[Notorious QED]

I'm a physics/math double major with a minor in CS applying to CS masters programs for machine learning. My first letter is from my research advisor who I've been working with for 2+ years. The second is the department head who decides who decides who gets awards in the physics department; I literally hit every award and I also aced an upper level class with him, so I assume he thinks positively of me.

I have two choices for my last letter of rec:

1. A math professor. Basically took a class and did well. She's actually my research advisor's wife, and my advisor has told her about me. Probably doesn't help the letter though.
2. A collaborator in my research group. She's a faculty member in the physics department of another university and I've been presenting my work to her ever since I started my project. Honestly, she's not crazy about me, but she said she is willing to write a letter. I feel that it would be kind of lukewarm because, like I said, she's not crazy about me.

Because I'm applying for ML, I thought it might be beneficial to get a letter from the math department. But that's would not be a research letter. Then again, a letter from my collaborator would be a lukewarm research letter. My advisor is going to write a very good letter, so I also worry about the contrast between the two letters--one being really awesome and the other being lukewarm.

All that being said, who is the better option?

Edit: typos

Edited November 1, 2018 by Notorious QED

[Teaching Faculty Wannabe]

On 10/31/2018 at 9:51 PM, Notorious QED said:

All that being said, who is the better option?

In my opinion, I would say the math professor is the best bet. It may not be as strong as your other letters, but I think you want someone who likes to write a letter rather than someone who knows you longer but doesn't like you as much.