Well I'd say to look at the role you need them to fulfil. That sounds a little too vague and abstract maybe, but the way I see it is to either make them as hard to break as possible, which would ideally mean 5 wide and as deep as you can afford (say 6 or 7 deep?). Preferably with a general or captain in there with good protection for that 3-dice break test. In that case Swordsmen would be a good choice, but maybe spears also as they are cheaper now.
If you want your infantry to cause as much damage as possible you'll want to put them in horde formation with a priest to give them hatred, in this case Halberdiers would still be the optimal choice. I played a 1500pt tournament this past weekend and took one Horde of 40 Halberdiers, incl. Priest. In the first game they faced a horde of Dwarf Warriors, unsupported. Naturally they were destroyed, and broke after a single round of combat. So in this case I had to support them better, a unit 50 or 60 strong would have made little difference as the Dwarfs would have outfought them anyway, maybe then in two or three rounds of combat.
In my second game they were charged by a Hydra and 20 Witch Elves (boosted with an add. attack by a Cauldron) and obliterated again. Again I made the mistake (likely at list building level too) that I had them facing off against superior points/power unsupported. Maybe a bigger horde might have worked, as the Dark Elves are all about hitting and breaking, what with Hatred and the Hydra's breath weapon. But maybe not..
In the two other games the unit was stomped to oblivion by Foot of Gork or dragged to their doom by Dweller's Below.
All this long windedness
leads me to believe that, yes, a horde of Halberds with Priest can put out a lot of hurt but it definitely needs proper support (duuhhh), either with good missile/war machine fire to shut down enemy heavy hitters and/or whittle down enemy blocks or by having more combat blocks ready to charge in (or be charged) simultaneously. The larger a horde, the more points you miss out on that might buy you a cannon, helstorm, a few flaggellants, greatswords or whatever. However, too small and your horde will be rendered ineffective easily. I'd say 40 is smallish for Halberds, I'll take 50 next time or just take 30 or 35 Swordsmen in a bus formation instead and have the killing done by knights/flags/greatswords/artillery.