the President's menu of options looks very interesting. There's no way he can comply with all three laws: 1) Taxing to raise revenue X; 2) Borrowing to raise Y; 3) Spending in the amount of Z > X+Y. (I'm assuming that other means of raising revenue, such as selling Alaska back to Russia, or invading Saudi Arabia and selling its oil to China, have been rejected as preposterous.) So:
1) Taxing beyond X would amount to an unconstitutional assumption of the power of Congress to tax;
2) Borrowing in excess of Y would amount to an unconstitutional assumption of the power of Congress to borrow;
3) Spending substantially less than Z would violate Section 4 of the Fourteenth Amendment.
Under these circumstances, I read both Professors Tribe and Buchanan to be saying that number 1) is somehow worse than 2) or 3), while I read Professor Tribe to also be saying that number 2) would be worse than number 3), while Professor Buchanan is saying that number 3) is worse than number 2). I'm less interested in the specifics of their agreement and disagreement than in the shared assumption that runs through all of this--namely, that where a President's only choices are all unconstitutional, some of these choices are more unconstitutional than others.
That strikes me as probably right, but it's worth noting that there's nothing in the text of the Constitution itself that states this principle. Moreover, I am not aware of any well-developed case law, historical practice, or scholarly literature addressing the question of which constitutional violations are worse than others. Maybe the generation of careful thinking about this question will be a beneficial side-effect of our government driving the economy over the cliff.
In my view, the question comes down to whether appropriations bills are mandatory or merely grant permission to spend, which may vary from bill to bill, and the time order in which the debt ceiling and appropriations bills were passed. The concern about raising taxes relative to the other options has something to do with the notion that this involves a less bounded form of discretion than the other options.