Higher downpayment on mortgage at ~6% rate, or put that money toward 401k instead (no matching)?

Buying a house with a ~6% mortgage. Throughout the rest of the year, I can either invest my extra income by:

a) Post-tax using it to pay down the mortgage principal, OR

b) Pre-tax into my 401k (no matching)

Which is a better plan and why?

(I have maxed my Roth IRA and paid off all other debts)