I've been working my 6600 hard for years and it continues to perform beautifully. One day, picking it up in the shop from a regular servicing, I spotted the 8900 and couldn't resist the larger throat. Now I have both and use the 8900 primarily but there are a number of things that the 6600 does best and I still use it. My recommendation would be to consider carefully the reasons for a change. Also, list the features you like best about the 6600 and make sure your new machine has them.