Your question stated a different way is Japan (low, stagnant rates) or Weimar (low rates followed by super, super inflation high rates).
If you had asked the question 5 years ago (2011) variable rates "won" but at a risk point that's pretty risky.
The way to "price" that under classical theory is to look at the 5 year rate as people locking that rate assume that's the highest "best" rate available.
Short versus long bond rate:
https://www.treasury.gov/resource-c.../Pages/TextView.aspx?data=yieldYear&year=2016
Technically, with institutions buying between the 1.8 and 2.3% rate, that's seeming to suggest that the banks think that rates won't increase beyond that spread. Problem is that when banks mess that up, you have a credit crisis. Even with 1% higher, 3.47% is still lower than your fixed quote.
In 2011:
https://www.treasury.gov/resource-c.../Pages/TextView.aspx?data=yieldYear&year=2011
2.3-3.2% were the rates quoted, so if you had "saved" to buy until today as a bank, you lost on the spread (you should have bought in 2011 not today).
Is the lower rate something that you are willing to risk considering this? That's why the banks are loading up on the debt because it's so, so cheap.
I don't have a mortgage anymore, but I still personally would do fixed rate as I don't trust economists very much having to deal with their sensationalism about growth. I also don't consider having a job necessarily a given at this stage in the profession (even though I'm government now).
(Those of you who actually do the math, there is an economics theory called Black-Scholes that posits rate convergence where that argument above does not apply. In practice with the time series evidence, that never worked out and almost collapsed the market via LTCM when those economists tried to put that into practice. Never figured that a revolution would occur...I believe in the classical theory that the interest rate reflects what rational institutions would hedge their interest rates again so someone buying a 10 y bond at 3% does not usually believe that the increase in a decade would be more than 3% + the National GDP growth rate.)