Regulatori makes some fantastic points and I support everything he says.
The 3.0L in the SC makes for an extremely reliable car, these same engines unmodified have made the Paris to Dakar rally. Shows how really robust they are, try that in a 300ZX TT! LOL...200K miles is easy...Porsche loves FRESH oil BTW!!!
Really the 300ZX is a spendid car and even though they are the same price, they are a million worlds apart. To be truly honest I am a Porsche 911 owner and lover and if I had to choose between a 300ZX TT and a 911SC I would honestly choose the 300ZX. Its modern, it had modern suspension and brakes and can be upgraded to be extremely fast without sacrificing reliability.
The 300ZX is a pain to work on and the 911SC is far easier to wrench on. I can pull the engine in my 911 in 4-5hours by myself. And what people say about 911 Porsches being expensive, is not true. Everyday consumeables like tune up parts, brakes, etc etc, are cheap...same as owning any Nissan. Forum support for both are excellent - which is a bonus.
So it comes down, to personal preference and which car makes you feel passionate, no forum could ever answer that, that comes from deep within your bowel.
Once you drive a 911...it is simply addictive. Check http://www.pelicanparts.com, they are the largest Porsche forum in the US for sure and have an EXCELLENT selection of enthusiast pampered cars for sale, also check http://www.rennlist.com (the classifieds).
Good luck.
Yasin