High IF Journal From Another Field vs. Low IF Journal In My Field I will be submitting a manuscript I have, but I am having a hard time deciding between two journals that are both suitable for my paper's topic.

Journal A has an impact factor of 5.6 but is a field-specific journal in a field I am not a part of. Researchers from my field publish in Journal A rarely. Journal B, on the other hand, has an impact factor of 1.6, is a general journal in my discipline and has a section that publishes papers from my field exclusively. Researchers from my field publish in Journal B more frequently. However, Journal B is still not a standard/well-known journal in my field. Which one would be more impactful in my CV (for graduate school applications) given that I am an undergraduate student.

I know this is hard to answer without knowing my field, but this question essentially boils down to whether a higher IF journal from another field is better than a lower IF journal that is more relevant.
Let me suggest that the IF of the journal overall is less important to you than having your paper seen by the people who need to see it. I assume you are interested in citations of your work. You won't get them if the people in your field don't have ready access to your paper. 

I think that a specific journal is a (valuable) benefit to you, independent of IF, which is an overall measure that doesn't really impact any particular paper. 

This is especially true for hard-copy journals that have wide subscription within your field. Your paper will naturally appear before suitable readers. But even for online publication, your readers need to have ready access to at least the index of the journal in order to know it exists. 

The above is opinion, obviously.