I've driven a 996 GT2 with a mild chip (otherwise stock). IMO you would have to do a lot of work to an NSX (i.e. more than just a power-adder) to get to the same level. I'm not saying it can't be done, but it'd take some doing. If someone handed me ~$70-80k and said "buy a fast car" a GT2 would be very very high on the list.
Ze Porsche can be set up to be more forgiving, the car I've driven is my dad's. We've had it on the track several times. He's ended up backwards once so far, but nothing tragic. Just the usual mid/rear engined/lots of power points of warning: lift-off oversteer, boost delivery may be sudden, it's hard to recover any type of tank-slapper you get into, careful in the rain, etc. I think people get caught off guard by how quickly these things gather speed. I've accelerated up an on-ramp merged, looked at the speedometer and jumped on the brakes because I was accidentally going ~150mph. It felt about like 80... Carry that into a turn or not realize you need to brake as hard as you need to and you could definitely get in trouble.
I cannot really even compare the power delivery of that car to my stock-engined '92 NSX. The Porsche has an obnoxious amount of torque from almost any revs and the top end is ungodly. I'm convinced it is partially fueled by the souls of the damned that the intake tract sucks from another dimension...so you might want to keep pets and small children out of the garage at night.
The NSX sounds better IMO, and I honestly like the way the NSX rides on the street much better (and you can see out of the back of the NSX a bit better). The GT2 has a pretty firm suspension and very wide tires so the ride can be harsh and it does tram-line on imperfect roads if you have even a slightly aggressive alignment / sticky tires. That said, these are the same reason that it has near-zero body roll and about as much grip as a non-pro-racer will know what to do with. Ride-quality aside, it's actually a very practical car. A large "frunk" and no back seats means it has copious storage space, and it probably goes without saying that all of the electronics work very well.
One more thing, get the sports seats if you get one. The normal seats really suck compared to the NSX seats. I really don't find them very comfortable and supportive.
So to answer your question:
If you'll end up on a track a bunch, or just want a speed implement, get the GT2. It's dry-sumped already, and will put up with any abuse you can throw at it.
If you just want a car to drive a lot on the street, and like the attention, IMO boost your NSX.
