Lemmy.world has open account creation, while Lemmy.ml requires an essay to create an account. For that reason, I believe .world has more users. But you can access content from either one no matter which you make an account on. Similarly, you could pick another instance or even host your own and still see and post content to other instances in the Fediverse.

I’ve tried out Mlem and Memmy, and so far I have preferred Memmy. The dev has been improving it at an insane rate too, so if you don’t love it yet, just wait a couple weeks and check again.

I think some instances have profile pics disabled? I’m not entirely sure though.

Fuck u/spez, indeed.

Bear in mind most apps have only been in development for a couple of weeks at most and they're all prioritising different features. You can always log into the mobile site and upload an avatar that way if you can't yet do it in whichever app you've settled on.

(For your other question, as claymedia explained lemmy.world and lemmy.ml are just two instances of many that are both running the Lemmy software, with different signup rules and different communities hosted there. But since you can access the communities from wherever, just pick whichever home instance you like and get subscribing)

lemmy.world is the most popular so far. lemmy.ml is run by authoritarians/tankies. I myself am on kbin.social. beehaw.org seems pretty popular, too.