They measure different things, but both tell roughly the same story, in terms of how many people are on servers.
My advice is that your best shot at realm level is Argent Dawn Alliance. I’m sure you can find guilds to your liking on many realms, but I do love AD, where people still /salute and /thank and buff in the world more than I’ve seen on all other realms combined. I’m told that the Horde there, while smaller, also has a good community.
Trade chat is … um, interesting at times. Or not intersting. Personally, I like it.
You might also investigate Communities and Discords to find other people. Communities in general seem to be a bit of a tangle at the moment, with no way to find out what’s what, but Alliance-side I know that Calm Keystones and Scared of Dungeons offer cross-realm communites for doing dungeons, and there’s a Calm Keystones Horde side as well, and the Fail Train Discord goes dungeon and raid runs both sides.
