if you can tell the future with 100% accuracy sure it would be an easy decision. and yes, for some buying down a 1-3 points can absolutely make sense.  

It will improve accuracy in a stressful situation.

Although you could probably find better investments on paper, vacancy & turnover expenses are some of the most costly parts of owning investment property, and very difficult to forecast with perfect accuracy.

You may get a tenant that in the heat of the moment leaves a review that doesn't represent the true accuracy of the business.

@Jonah Hartsburg The FCRA says you must establish and implement written policies and procedures regarding the accuracy and integrity of information you furnish to a CRA.

You will get the accuracy you want while saving money.

Can't really comment on the difference in accuracy between the both though.

I'm unable to get consistent results for me to assess accuracy... 

I spoke with a sales rep from the Propstream team and they said that their sale price estimates (for non MLS users) are usually within 1% accuracy, but they didn't provide any way to verify that claim.

Focus on the accuracy of the CAP and if 6.125 works for you then plow ahead.