Depends on the program. Helmet schools, probably the bowl win, give yourself some momentum into next year, and inflated ranking to help you, and you'll probably have another great season shortly.
For other schools, where everything has to line up perfectly to even be in position, probably the CFP blowout loss. With MSU's the caveat is that we had just won the Rose and Cotton Bowls the previous two years. And no matter what I'd take a CFP blowout over a Cotton Bowl win. But if it had been 25 years since MSU had been in the Rose Bowl, I don't think I'd trade that for a CFP loss.