I am applying to math REUs in the United States, which require letters of recommendation. I wanted to ask my faculty advisor for the letter - I get along well with them, they have seen my progress over my entire college career so far, I ask them fairly many questions about graduate school, and they were able to get me a grant to attend a math conference. They are also professor emeritus at my college.
However, I haven't dealt in real math with them, in the sense that I haven't been taught by them or done research with them.
Thus, while my advisor would write me a good letter, I think it would not be from the standpoint from someone who has directly witnessed my mathematical skill?
I am planning on sending one letter from a professor who has taught me in an intensive course. Is it a good idea to ask my advisor for the second letter, or should I ask a professor who has taught me instead?