Analyzing a 30 year mortgage vs a 15 yr

[David Kerr]

I have thoroughly confused myself... What if the numbers work on a 30 year mortgage but don't on a 15 year mortgage.  As far as cash on cash, you would't be receiving as much passive income because more would be going to mortgage pay down. What is the best practice? Sorry if a rookie question...

[Joe Villeneuve]

If I can get a 40 year, I would take that.

There are two factors here with regard to my cost and income:

1 - Down Payment - The lower the better.  This is my cost to buy.  In fact, this is my ONLY cost to buy since my tenant pays the rest (mortgage).

2 - Mortgage Length - The longer the better.  This is my income through cash flow.  The longer the loan, the lower the MP.  The lower the MP, the higher the CF.

Erroneous assumption:  Paying higher DP improves CF, or turns negative CF into possitive CF.

Why?  

If you have negative CF, and put up a higher DP to get a positive MCF, all  you've done is pay all your negative CF upfront. 

If you have positive MCF to start with, all you've done by paying a higher DP is "buy" the added MCF.  Why do that when you're not the one paying off the mortgage?