Paying points up front for a lower interest rate... bad idea?

[Jason Howell]

I'm about to close on my fourth property and up until now, I have opted to pay whatever points were necessary upfront to lock in the lowest interest rate. It just made sense at the time: Go for the lowest interest rate, especially while rates are particularly low to begin with.

But I'm now realizing that if my goal is to buy and hold for the long term (which it is), it may make more sense to accept an interest rate that's higher while keeping that extra money in my account for future investing...given that cash flow is still positive.

Example: 20% down for a loan of around $66k... IF I chose to pay $1200k for a 30 year loan at 5.1%, how can I work those numbers to see how long I will be paying on the property for that to equal or surpass what I get if I choose to pay $200 for a 30 year loan at 5.5%. 

Anyone happen to have a formula you use to determine this on the fly? Or maybe you believe that, all things considered, it rarely makes sense to go one way over the other.

Thanks all!