Ulm Crossbowmen use Arbalests, which are really just big cranked-up crossbows.
I just finished a sort of experimental SP game where I played Ulm. I saw my PD tested many times, and Ulmish PD definitely packs a whollop. Offhand, I'm not sure there are any better, save for those of Jotunheim.
Anyway, getting a Crossbow per point up to 20 and then an Infantryman per point after that matches what I was seeing. Lots of bolts flying across the screen and a few of those little Ulmish blue-clad guys chasing after them towards the enemy.
Also, iirc, you get your second comm at 20, not 21. You'll notice Ulm gets its Commander (or the Priest, but I could swear it's the Commander) at exactly 20 PD.
And I think Jotunheim is an absolute waste after 20 PD. 1/2 a Hurler? LAME.