First of all, 996tt's can be had w/ extremely low miles,(2001&2002), for more like $110k. Look on ebay, there are a ton of them. They sold a LOT more of those than planned, so difference between 993& 996tt's is more like $30-40k. 993 turbos are also twin-turbos, almost zero turbo-lag and 400hp. Biggest difference is that 993's are last of the air-cooled cars.
I've always been partial to 993tt's due to styling/ smaller size, but have to admit that 996's have been growing on me. (Porsche is clever like that

). Both cars have mind-blowing performance, (0-60 in 4sec., 190mph for 996), big reserves of safety, lots of comfort and BIG maintence bills when it's time. A clutch on either car is approx. $3500 parts+labor, and they eat clutches, tires and brakes if you drive them. All are expensive. But if he's got the $, I say "go for it!" A big part of price difference between the cars has to do w/ the fact of the 996's being under factory warranty. Watch for prices to drop significantly when the first ones run out of warranty.