Well, friends, after sleeping on it, I have just pre-ordered a 6600 from the good folks in Austin. The lure of two SCUs finally won me over.
Eric at FRS was exceptionally gracious and professional. I feel like when I call Austin, I'm calling friends. He explained to me the lead times and the fact that trade up pricing is not yet available (until the market settles, he said). Took my fully-refundable deposit.
Planning on trading in the 6500 Limited Edition this autumn. It's been an amazing radio that has helped me win my section in two CW Sweepstakes, get WAS on 10 bands, 279 DXCCs, and over 300 grids on 6m.
Do I "need" it? No. The 6500 could serve admirably for years to come. But with me hanging transverters and such on it, the desire to simplify life with the 6600 made the decision clear.
By the way, Eric mentioned the GPS module remains the same for any 6000-series that can accept one. Good to know.
Let the good times roll!
PS - Some "peer pressure" did help the decision. You know who you are ;-)