bkennedy,
I too struggled with your decision.  I had a 445 with the MC519 and powerflow head and loved it.  I wanted to upgrade, but the 2210 did not have a collection system.  I wanted the 2-speed hydrostat and higher capacity load on the 2210, but wanted the X-series collection system.  
I studied the situation and decided I could make the MC519 cart work with the 2210 since the mower decks are identical and the powerflow head would fit the 2210 decks.  All I had to do was mount the MC519 cart.  
If you search on my user name for my previous post, you will find one on my MC519 conversion.  I ended up making a hitch that bolts to the front of the cart and works with either the links or a quick hitch.  I've been real happy with it and I have the added benefit of being able to carry the cart and dump it from a height of about 3 feet.  Let's you pile the collected material onto existing piles.
Let me know what you think.
John