I don't know much about the 5200, but I have the 6600 and there are a few features that I found to be invaluable for quilting.
One is the integrated walking foot. I thought I would only use it for SITD and binding, but I find I use it for most of my piecing - especially when piecing long strips and flannel. The reason is that with the dual feed you don't get "bowed" strips or "extra" material at the end of the strip because the top fabric didn't feed as even as the bottom fabric.
The other feature that I NEED for quilting is the needle up/down at the touch of a button. I can touch the button and every time I stop the machine the needle will stop in the down position. Almost essential when FMQ'ing.
If the 5200 doesn't have these two features I would spend the extra $$ on the 6600 because these features are worth it.
You might want to ask your dealer if they have a used 6600 for sale. If it is a good dealer that took a 6600 in from someone who upgraded to another machine - the dealer has given the machine a complete service, will usually offer a limited warranty, may offer a class or at least support, and you'll get the features you need for less $$ ... possibly even cheaper than the 5200.
Good luck!