Master of Nine is insanely good for Crusaders if you can manage to satisfy the prerequisites. Unfortunately, the prerequisites are about as insane for a class with only 3 disciplines as the benefits. If you go that route, take a 1 level dip in Swordsage or Warblade to get the maneuvers from different disciplines prereq.

As for why Master of Nine is so good? 5 more maneuvers readied. Additional maneuvers readied also increase maneuvers granted. Think about it, 10 or 11 maneuvers readied at once, all but 2 or 3 of them granted instantly, and the whole lot refreshing on its own every 3 or 4 rounds. All that on top of being able to pick from every discipline and getting huge numbers of maneuvers known to pick with. Your bag of tricks just expanded hugely and got a lot more flexible and reliable, and you can burn through strikes, boost, counters, and miscellaneous stuff (some maneuvers like Shadow Stride use move actions) as fast as you can spend the actions and you still won't run out before it all refreshes.

If you don't want to spend all the resources required to qualify for MoN, though, I'd say just stick with Crusader. It's a good class all the way through, you don't want to stop maneuver progression, and all the other ToB PrCs are either not suited for your character or not very good.