Hard to say. After playing one, you're interested in the other. Each faction has a few unique units. Maybe more than a few but I need to proceed further in the game yet (not yet quite 1200, I think I'm at 1180 atm).

Must say the Danish appeal a lot to me. I like infantry shock troops, and their axemen (huscarls, viking raiders) are great. The beat every spear they meet. Unfortunatly, I didnt meet a lot of sword-yielding infantry as an enemy yet, maybe because I'm only warring with poland and HRE atm, they seem to use mostly spear.

And I hear people saying that the Danish arent nice to have as an enemy, probably because of their axe-troops.

But I'm looking forward to trying billmen and longbows, and highlanders, and the French cavalry..