the 8900 must sew REALLY well in order for you to consider giving up your 6600. I just love mine and take it everywhere I sew. I know others think it's too heavy, and buy a smaller machine, but I just love how it sews. If I had a smaller/ different machine, I'd always wish i were home sewing on my 6600 instead of at retreat or at sew-ins. Keep the 6600. It will come in handy, or you can use it as your travel machine. (I plan to use mine for everything until I'm too weak to lift it in and out of the the tutto bag.
Congrats on the new machine.