It depends on what kind of niche in your application you're trying to fill.
At many schools, you need a certain number of letters from people who have formally taught you in the classroom regardless of how many research experiences you've had. If you haven't filled these slots yet, you need to go to your professor first.
If you have fulfilled that requirement, research letters are much better than additional classroom letters. However, what you really need to do is ask your post-doc to draft a letter, and have your PI review and sign it. Neither your PI nor your post-doc should be offended or surprised that you ask for this, as they should know that you need to be faculty level to sign a LOR. Two of my letters were drafted in this manner and I didn't even to request it, it was just assumed that this was the only way to go.
Hope that helps!