On most of the "combo" edged knives I sharpen, the serrated part hasn't been used. And the answer is quite simple.
Most guys want to "save the edge" for a real or imagined emergency. And most folks know that when a serrated edge degrades, it's hard to bring them back.
I've seen the 'effects' of using those rat-tail files in sharpening the serrations from the front side. It's almost impossible to hold your hand straight and level at the identical angle for +dozen serrations. Most of these repairs simply ruin the knife.
The serrations should be repaired from the back side. Using a finer grit stone, the sharpener should make several down-strokes--like he was trying to straighten out the little "fingers" of the serrations. When his stone passes begin to quiet, he can use up and down strokes. Then he should polish this repair using the same procedure.
As for this "which is better" routine, you have to ask yourself if you need serrations. And the fact is, most of you don't.
If you are an over-the-road trucker, a sailor or a working cowboy, you probably will need to cut something slippery or wet on your job.
(Sharp knives cut seatbelts, so I'm 50/50 on first-responders.)
The real problem with serrations is neglect. Dull knives don't cut, but dull serrations can rip. A guy outfitted with a combo knife, or a knife that is fully serrated tends to leave his knife in a poor state of repair.
It always baffles me why a hunter pays +$1K for a rifle, almost as much for a scope and then uses a 5-dollar knife. Same here.
If your job (or life) depends on the emergency ripping power of serrations, then for pete's sake, learn how to sharpen the plain portion of the blade.
If the serrations are part of your job, you might think about carrying a fully serrated knife (in pristine condition) and keeping a fully plain, sharp knife as your EDC. Knives don't take up much room, and most of us TFL members have pockets full of toys.
Letting a serrated knife go dull--or carrying one so you can let the plain portion go for longer periods of neglect--is like leaving an engine in need of a tune-up, but then strapping in a turbocharger to make up for the loss of performance.
BTW, I don't own a serrated knife, and as a distributor, I can buy any knife I want for next to nothing.