Per Google Maps, Nana BTS is 450 meters from the JW. Phloen Chit is 400 meters, so pretty much a wash. The supposed dangerous highway entry/exit is on the other side of the road, so you won't have to deal with it if you're just walking up from the JW to the BTS entrance at Phloen Chit.

I really don't understand the sketchy bar area. Sketchy street food vendor, sure. Noxious air pollution under the highway? Most likely. Danger? Not really.

As CJ states, if you're heading one way go to Nana, if the other walk to Phloen Chit. If you're afraid of crowded sidewalks, dicey pavement, dodging motorcycles and scooters, weird foods, street people or poverty in general, then going to Bangkok will not be a good experience for you.