I prefer the overserrated sections because there is less wear on the guard ledger surface. That means the guards last longer. Supposedly the underserated knives give a better cut in grasses with less horsepower needed, but I stopped using the horse and mule a LONG time ago. You can use non-serrated sections just as well if the field has rocks in it that you are growing in addition to the hay. Underserrated sections are supposedly more easily sharpend because the cutting surface is continuous, but I don't sharpen, I replace. Switching over to bolts from rivets encourages fast knife replacement.
The fact is, I buy whatever is cheapest of the on sale or opened package (missing content) stuff at TSC. I carry a spare cutter bar within the mower cross frame tube so changing is done fast when necessary.
While you are at it (presume its maintenance time), check the belts, hold down clips, reel tines, tire pressures and sickle drive bushing. You can still buy last year's parts at last years prices instead of waiting until June 1st...