Would it be better to invest or pay down mortgage?

[Mano]

I have somewhere in the neighborhood of 150k sitting in a low interest account (~4.5%), and want to move most of it. My job is fairly demanding on my time, so I am looking at either putting most of the money into index funds (spy, dia, vti, qqqq, etc.) or paying down my mortgage.

I owe a little more than 300k on my home, with a 6% fixed rate mortgage (~28 years left). Home value is probably currently somewhere between 600k - 700k. I am 40 yrs. old, married with 2 small children.

Which option do you all think would be better in the long run?