The 2006 will be under warranty, so you'll have zero out of pocket costs to run the 997 for a few years. I'll bet any 993 TT will need something at this stage of the game. The newest 993TT is almost 10 years old now. No matter what people say, they all need something (or will need something in a short order of time).
The 993TT will relatively raw yet as compared to the 997 and was Porsche's last air cooled Turbo. You'll have sounds and feel that is missing from the water cooled cars.
Are you considering a 997 S or just the 'base' 997? Even being 9-10 years old, the Twin Turbo will still be producing more hp than even the "S".
I personally would choose the 993 Twin Turbo if I wanted a toy. I'd take a 997 if I needed a 911 for everyday. Both good cars. I love how the 997 looks.
Jay
90 964, 84 3.2