It all depends on the stage and financial situation of yourself

I use 15yr in the beginning since I have no intention to expand. This way pays less interest.

Then, I want to expand and so refinced to all 30 yrs. (accumulate more downpay and reduce DTI)

Now, I reached the limit of # of loans (no more loans can be approved). All refinanced to 15 yrs (pays less interest, still positive cash flow).