I think Sun Country is absolutely awesome and will always choose them over NW when their schedule meets my needs. The reason: cheap first class travel. Last year, for example, I flew DFW to MSP on SY in first class for $230 one way (with no restrictions). NW's one way Y fare was over $700 (so was AA). Under such circumstances I cannot fathom why anyone would ever choose NW unless they are just hopelessly addicted to Worldperks.

SY first class is quite impressive: 39" of pitch and standard width FC seats on a brand new 737-800. They served a hot meal. The meal was not quite up to the standard of some legacies' FC meals, but who cares when you are paying only $230 for FC? They also passed out hot towels (actually paper "towelettes" but still a nice touch).