I would stay away from an automatic unless it is an absolute necessity (unable to drive a stick, due to some disability, etc.); besides being generally less enjoyable to drive, they are detuned from the factory, and have a different cam profile.
The 1991-1992 models suffer from snap ring issues on manual transmissions, this is a manufacturing defect which can result in a failure that will immobilize the car, and will cost around $1500 in parts, and $1500-2000 in labor to fix.
1993+ cars have dual airbags, instead of a driver-side only airbag, Acura fixed the snap ring problem by changing the manufacturing process on transmission cases, the cars also feature cup holders, and some other advantages. 1994 is the last year of the hard top, and has redesigned (larger) wheels, and an exhaust with dual round tips instead of oval tips.
1995+ years have removable roofs, the NSX-T, but 1995 is the year Acura switched to OBD2 diagnostics, so perhaps harder to modify extensively.
1997+ nsx's have 1" larger diameter brakes, 3.2L engines instead of 3.0L engines, redesigned exhaust manifolds, and a 6spd manual transmission instead of a 5spd.
2002+ nsx's have a face lift with fixed headlights, and have 17/17 redesigned wheels, as well as different side skirts, rear fascia, and tail lights.
2004-2005 models have a really cool shift knob.
