Neither. My understanding is that it was developed to fill in the gaps that LEED has.
LEED is really meant for buildings with human occupancy (ie. not un-manned pump stations). The ISI certification is meant to cover everything from roadways and pipelines to buildings. LEED is also very prescriptive in its rating system whereas the ISI system appears to be a bit more subjective in that you can still get full points if you are truly doing the absolute best that you can do environmentally.
Most importantly, certification of projects is cheaper than LEED.
There's been a bit of buzz about it in the consulting industry lately, and I figured since I can no longer become a LEED AP w/out LEED project experience (which aren't very numerous due to the fact that buildings may be constructed to those standards but never certified as LEED due to cost), this ENVISION certification might be the next best thing.
I'm not going for the USGBC GA certification because to me carries little to no weight at a very high cost.