4 wide, 1 rank, lances.
Maybe 3-wide if you're stretched for points, but 4-wide will maximize frontage and attacks.
That is, assuming 20x20mm bases really are 20x20 (they're not), or at least you play it that way.
Lances give you +1 armor save in close combat, halberds don't. Now if you could take greatweapons...that would be a tougher call.