The two comments don't conflict, because they're measuring different things. Your 90% is loan to value at creation, the note balance against the property value. Investment to value is the note buyer's number, their purchase price against the property value. If they pay 82 cents on a $288k balance, that's about $236k against a $320k house, so their ITV is 74% even though your LTV was 90%. The discount is partly how they get to their ITV. That's why the broker can want 80% ITV and still look at your file.
The balloon helps and hurts. It shortens duration, which the buyer likes, and it concentrates most of the value in one distant payment, which gets discounted hardest. If your goal is the best price on a full sale, run both structures through the buyer's target yield rather than guessing. The 10.5% coupon on a 20 year full amortization usually prices closer to par than 9% on a 30/5, because more cash arrives sooner and the coupon gap to the buyer's target is smaller.
What will actually decide this file is not structure. It's the valuation the buyer orders and the compliance file. They'll get their own drive-by or full appraisal, and seller-financed sale prices often sit above what an appraiser supports, which recalculates their ITV against you with no warning. And if the buyer occupies the home, a federally regulated originator issue or a state licensing defect in how the loan was made can make the note unsellable at any price. Get that reviewed by a licensed professional in your state before closing, not after twelve payments.
Also price the partial. Selling forty eight payments and keeping the balloon may beat every full-sale number you're comparing.