<<So the conclusion for us is the self-imposed addional savings in the form of coughing up the tax money now.>>
Really? Do you also have a zero-exemption W4, so that you get a large tax refund every April?
If this is your only reason, it doesn't make much sense to me. You are paying a certain tax now, in the hope that doing so will prevent you from having to pay the same identical amount in the future. Does the phrase "time value of money" ring a bell? What if the tax rate in the future is lowered? (yeah, I know....fat chance!) You will have paid a large tax now, and saved a smaller tax in the future.
One of the cardinal rules of cash management is to delay paying out money to as far into the future as you can.

Regards,
Ray