I have read both of their description and they seem to do the same thing. So which one should I use.

You could try asking The Castle this question, more than likely you'll get an answer.

Personally, I run both of them together, as well as populated casinos.

Honestly, I don't think they do quite the same thing though.

Populated wasteland mostly adds NPC to towns and such to spruce them up a bit as far as I know. While IWS adds spawn points and patrols so that there's more combat potential in the wasteland.

So Populated Wasteland is something to add flavor and live to settlements and places you visit.
IWS is about making combat spawns a bit more random and larger so that you get a bit more out of "random" combat.