Selecting the best referee for CGS M reference assessment

So my options are:
Current PI (under whom I'm writing the CGS-M proposal) I've only known for 5 weeks. But as a complete beginner to the field, I've learned new skills from the ground up, and the PI was happy with my progress/ the mini-project I completed.

Versus the 2 other potential referees I've known for longer (1 year) (1st is my post-doc mentor for an undergrad research project and the 2nd was my professor for a undergrad course). Any thoughts on who I should ask to complete the reference assessment?