Have both the ns-2 and the decimator and use both in the one rig. I ended up favouring the ns-2 for the effects chain, it works great at keeping the noise from the pedals quiet, using it in the loop mode, and I favour using the decimator in the effects loop of the amp.
So, ns-2 to keep your pedals quiet and decimator to keep the amp quiet. With the ns-2 off, I get a loud ssssshhhhhhhhhh sound and with the decimator off, I get a hhhhhhhhhmmmmmmmm sound. With both engaged, I can have whatever amp on 4 (fucking loud) stand in front of the amp with the volume up on my guitar and nothing, just the slightest of hums, almost at whisper levels.
The main difference for me between the both is the sustain factor. This is where the decimator kills the ns-2. Notes do not get chopped off with the decimator, thats why I have it in the effects loop. The ns-2 is not a tone sucker for me, its job is to keep the pedals quiet and it does this brilliantly. Some people use the ns-2 like a normal pedal, going into the in and then going out from the out. I have not used it this way so I cannot comment, but the loop function works well.
The decimator is my weapon of choice in a fight. The thing weighs a ton and can come in handy for other things like a chock for your car or an anchor for your boat. Looks damn sexy covered with all that chrome too.
At the end of the day, I would say definately grab the decimator, but if you have the cash, grab both.