I think you will find it challenging to leave your 6600 behind. Mine is such a workhorse and I find when I want dependable production without too much fuss I always return to it. I upgraded to the 8900 because of the throat space but have found it to be finicky. As already noted, it "eats" fabric; it prefers blue bobbins; it prefers colour-tipped needles; the needle plate used makes a difference. I have also spent a small fortune buying add-ons such as the dual-feed feet since the dual-feed feature is not included on the machine.
My suggestion is to really think about why you want to say goodbye to your 6600 because you will miss it.