I've never played BKBs but I have played XL and would definately reccomend it. I'm still playing as the Cumans and can remember some of the epic battles I had with the Golden Horde, two evenly matched cavalry armies fighting with their 5-6* kings leading the way is definately something that should be experienced.

Also the new economic system seems to be more reflective of the times as 20,000 florin incomes seems to be impossible. For most of my campaign (Cuman, expert) I had to struggle through with incomes of 300-400 florin, carefully plotting my expansions to ensure I did not lose men who couldn't be replaced.

The XL mod was definately the most challenging mod I've played and I highly reccomend it.