According to my calculations, 11.67 halberdiers are killed each phase, compared to ~9.72 swordsmen, the difference is ~1.95, swordsmen therefore lose almost 2 models less, right? Taking account cost/model, Halberdiers lose more "points" each combatphase. So even though you can buy more Halberdiers, those will die at a rate that cancels that advantage. The pro with Swordsmen is that the give away less negative CR in this situation.
Negative CR from the steadfast unit is something that is very important if you are flanking with killy troops, which is why swordsmen are better than Halberdiers in a bus IMO.