Which among an LoR for startup work and a "good student" LoR, will be more effective? I am applying for Master's program in CS. I have the following two alternatives:


An LoR from a startup founder for significant and relevant work. The possible problem is that the entire team consists of final-year undergraduates including founders, however they are from a relatively well-known university in the country that I am applying to (USA).
An LoR from one of my professors with whom my interactions have been only during courses and labs which are not directly relevant to my area of specialization.


Which of these would be more effective as part of my application?

My other LoRs are from a professor with whom I have worked on a relevant R&amp;D project for almost a year, and one from a professor about coursework and labwork, which is relevant to the field.
If the startup is showing evidence of success (e.g., it's actually raising significant venture capital or making money from clients), then if it were me, I would absolutely go with the letter from the startup.  They may be have been undergraduates, but now they are a Small Business Success Story, and you are part of that story.  It will help you stand out as unique, capable of taking initiative, and capable of working on ill-bounded problems as part of a team.

If, on the other hand, the startup is still little more than a collection of nebulous hopes, dreams, and semi-finished software, the professor (who is essentially neutral as far as recommendation value goes) is a safer bet.