I am in the process of applying to some masters programs in machine learning, and need to make a choice between two letters of recommendation. One is from an undergraduate professor who told me they were happy to help, but my only interaction with him was one class I took senior year, and I graduated seven years ago. The second is from my current manager at work, who was a former academic whom I have a good relationship with. He knows that I am applying to full time masters programs, and that if I get in I will be leaving my job, and is still willing to help. He also knows the amount of effort I put in outside of work hours to work on machine learning projects. I understand that academic letters of recommendation are preferred, but given the fact that I graduated seven years ago, and that the letter from my manager will most likely be stronger, I am looking for some advice.
I don't think it matters too much if the academic letter is about a old relationship, or whether the work letter is actually from your job and not from academia. What matters is that the academic you interacted with has an excellent opinion of you - and that seems to be the case, given the many years in between! - and hopefully that the academic stuff you have been working together is related to your master. Similarly, your employer will have lots to say about your work aethics and dedication, and hopefully his academic background and/or your current job should be somehow pertinent to your masters.