There is no free lunch.

You will discover there is a difference in cost [either loan origination or other "garbage" fees or note interest rate] depending upon whether/not you demand FF miles and that cost will not be worth the miles. Make sure you ask for ALL the costs one way or the other before you commit.

Also the identity of the lender is immaterial because in today's mortgage market essentially NO ONE is a lender. All the lender does is originate the loan, package it together with other originations, and then sells the package to FNMA or GNMA retaining nothing more than the loan servicing which many times itself is sold off to someone else.

Good luck!