Hey Christian, few questions:
What interest rate are you paying the seller?
Not sure what you've seen from lenders in your market on long-term loans but I've been quoted between 6.5% - 7% for 30-year amortization at a 70-75% LTV. So you might be better off going with your credit union than to be saving 100-150 bps for a longer AM period and risking a lower LTV. Also some banks don't like seller financing, so you'd take a risk shopping this to the lenders that will offer 30 year AM.
What's your goal with these properties? Refi and hold long-term? In that case it may be better to take the try to find a lender who could provide 30-year AM since your monthly payment will most likely be lower. But again, if they give it to you at a lower LTV then your basis will increase and you may not be able to "pull out" as much cash.
I'd try to run multiple reports with these scenarios and look at them side-by-side and then make your decision. Good luck!