There are basically 3 types of nitrous systems. 1) The "dry system" adds another hole into your intake system and sprays nitrous from there. 2) The "wet system" taps into your fuel line and incorporates a "nozzle" that sprays both nitrous and fuel mixed into the intake manifold. This is a little safer than the "dry system" because each cylinder is receiving the same amount of fuel and nitrous, given everything is running i.e. no clog fuel injectors. 3) The "direct port system" is very similar to the "wet system" and by far the safest, but most expensive system. Each cylinder receives it own "nozzles" and nitrous/fuel is truly distrubed evenly. In my opinion, the dry system is the cheapest and easiest to install. If you're running only 35-50 shots, this is the way to go. The wet system would be next if you had a little more cash or want to be on the "safe side." Nitrous is always activated ONLY at full throttle with some exceptions. There are ways to fool the system into thinking its at WOT and have it has a push button has seen in FnF and Gone In 60 Secs. Also, if you get the Venom systems, you can change nitrous activation and amount with a PC.
The safest to run nitrous in a "wet" and "dry" system is to get colder plugs and retard your timing. You can retard your timing by adjusting your distrubator. Throwing in a fuel regulator is nice, but not needed if running minimal amounts. Also, the rule of thumb for a stock car is "half the horsepower and minus ten." I.e. d16 is, give or take, at 105-110hp *not sure*. Divide that by 2 and minus ten. 60hp minus 10 is 50hp. Thus, run no more than 50 shots to be safe. You can disregard that rule when you've upgraded fuel, ignition, pistons, etc etc...
Ah alast, I would go with NOS nitrous given its simplicity. NX is good, so is ZEX, but their a "tad" bit more complicated because of extra "safety" electronics. Also, my friends have had bad experience with ZEX and blowing their motors. Faulty injectors mostly. Given that, its not really the nitrous system that messed up their motors. So all in all, their basically the same except for price.
Anyway, hopes some of that mumble jumbo helped. happy tunings!