How do I run these numbers on a 15 year vs 30 year mortgage?

[Paul E. Drecksler]

Can you help me with this math problem?

$425,000 sale price.

$85,000 down payment (20%).

30 year fixed at 3.675% vs 15 year at 3.15%.

Difference in cash flow / monthly payment would be $809/month on a 30 year, which I could put right back into the property to pay off the principal faster. How long until I pay off the loan on a 30 year doing that, and how much interest do I save? Is there a spreadsheet or calculator that can help me run these numbers?

Background of this question: 

I'm considering a 15 year vs a 30 year mortgage on an investment property. It's a buy and hold rental. If I do a 15 year, I'd save $187k in interest on the loan but the payment would be $809/month higher. 

A 30 year mortgage provides more monthly cash flow that I could turn into other investments. Or I could just use the extra cash flow to pay off the principal faster, but at least I'd have the option with a 30 year if I needed the cash flow at some point, which I wouldn't have with the 15 year. 

So there's pros and cons to either and my particular scenario and objectives will determine what's best for me. However at the moment, I just want to run the numbers to see what the difference in total interest payout would be on a 15 year vs using the same difference in cash flow on a 30 year to pay off the principal faster. How do I run those number? Thanks for any help.