oh both mods are VERY GOOD
I played both and liked them, there's a good and bad side to both

BKB mod has many factions and a whole lot of new cool units, but no GA mode
XL mod has a good map and GA mode but not a lot of new units

But I would definently pick the BKB mod to be sligtly better, I got 2 mtw instalions on my pc, 1 for BKB and 1 for XL :)