re: Down payment on a home - how much is too much?

I spoke with broker last week about this. I was surprised at how little the rate advantage of 15yr vs 30yr for me. Because of this, we are going to go with a 30yr but plan on paying 41% more per month (close to what the 15yr min is). For the sale price and down we are looking at, it would be a 17yr payoff. Not as good of a rate as 15yr but we have option of lowering payment in case wife wants to stay at home, we have life changing event, etc