Guys, thanks for the advice (even though I wasn't soliciting most of it).
I'm aware of the depreciation curve, and I am comfortable with financing as an option to acquisition.
I redid my analysis and saw what I was missing: the difference in monthly payment between the two scenarios. The principal reduction approach comes with about a $100 more higher monthly payment, which is why my payoff at 48 months is better in that scenario. But I pay for that equity either way.
Since, in worst case, I can sell my current rig privately for $13.5k and come out about the same at 48 months, regardless of the tax liability reduction, I think I am going to roll the dice and wait til spring and sell it privately when the market (literally) heats back up, and I should be able to sell it for significantly more than $13.5k.
Because I paid cash for my old rig, I have the option to wait and sell. Should I have financed it, it would be more difficult to pull that off.