(Your loan was for a flip, and presumably for a business purpose, which means the San Francisco Federal Reserve bank rate also plays into this.

Here is the purpose of my post.

All in all, I would compare the two and see which one works best for you. 

Are you able to track down expenses for accounting purposes?

I realize the purpose of seller financing can often be to get the seller the price they want and as long as you're cash flowing on the deal you're ahead of the game. 

You really need to balance the opportunity cost of the interest you'll save with the returns you can make by investing that $400/month or however much you can scrape together.If you want to get real nerdy, you can download a loan amortization spreadsheet, plug in the extra payments, compare the interest saved each month versus what it'd be if you keep the loan as is, and discount the NPV into today's dollars. 

Any improvements you make need to have a comp available to compare it against. 

@Vania Castillo, A 1031 exchange starts with the sale of a property you have held for investment purposes. 

San Antonio is amazingly priced compared to every market and it is flaming hot because of the affordability.

What is your purpose of having a rental?