/6 build cost

This is my opinion, others may disagree.
Initial install would be cheaper and easier for Nitrous, then turbo.
However there is no "throttle effect". It is either full on or full off. Not really good for street driving, plus you need to keep filling the bottle.
My turbo setup is/was "old school". I did it in 1979. It is a draw thru, using a Corvair turbo. There are much better ways to do it, today. It all depends on your budget.
Another option is the Torqstorm supercharger kit. No personal experiance, but have heard good things. Not high boost, which lends itself to use on basically a stock motor. Slant 6 Supercharger Kits from TorqStorm

Pics of my turbo setup in this thread.
170 slant six turbo pics