I've tried all three, and would forget the Boss in this day and age.
Actually the vast majority of pros are still using ns2's in this day and age......... mastodon, metallica, killswitch engage, machine head, steely dan, dillinger excapeplan, slipknot, periphery, symphony x the list is huge and its still the most popular noise gate amongst the pro's. Tone is subjective but I dont think noise gates are so ill go along with the pro's sure a few use ISP but who even uses the mxr noise gates??
I've had the ISP and the NS2 and MAYBE the ISP is ever so slightly better but for the money it really isnt worth it. Were talking about the smallest amount of noise reduction and does it matter when youre playing guitar any way? For pedal hum and a hi gain there may be a
little noise but turn your volume down too and its gone and youve saved your self some money!
The things isin fairness I get you only need some thing in the front of your amp which saves on cost but if anyone wanted a noise gate for the input / fx loop the boss ns2 is £69 new (Cheaper 2nd hand I got mine for £30) and the isp decimiator g string ii is an extra £100 which is just robbery imo !!!
That being said if money isnt an issue go with the isp