Choosing a reference letter recommender
You want a recommendation from somebody who can speak to your strong qualities.  This means faculty members you have interacted with; the more substantial the interactions, the better.  However, you also want the interactions to have demonstrated your existing skill and future potential.

If you took a one-credit class from a professor and have had little interaction with them otherwise, they probably cannot write you a strong letter.  The one-credit class probably had more limited interactions between you and the instructor.  if you were very active in class, that would be good.  But if you were not an extremely active questioner and class participant, that means the professor will have very limited basis for evaluating you (especially since one-credit classes tend to have rather little homework on the basis of which you can be evaluated).

In contrast, a professor you have taken a full three-credit course from and also interacted with outside class will probably have a much better idea what your skill level and potential are.  The downside is obviously that you earned a lower grade in the class.  Unless you had a lot of in-class interactions with the first professor, I think you should get in touch with the second professor and simply ask them whether they can write you a strong recommendation.  If they say they cannot, I would go with the first professor.  Otherwise, I go with the second professor.