996 Turbo because lots of the M96 problems are absent.The appearance is so similar to the regular 996 that it drags it's price down. Eventually the classic car market will recognize the merit and support good prices Similar thing happened with 914-6 vs 914.
Here
Porsche 996 Turbo buying advice | Evo
Just a pity the 996 Turbo engine isn't offered in the Boxster.