Honestly for the new user thinking to join just picking something, anything is all that matters.
Each user has very little content on here right now, so if for some reason they don't like the way things are run on the one they chose they can easily choose a different one without losing much. Profile migration (i.e. copy all your stuff to a different lemmy server) is also something coming that I'm looking forward to.
As a sh.itjust.works user you can see almost anything in this network of websites (500+ servers) except lemmygrad and beehaw. Also the server admin here is pretty cool and seems to know what he's doing, so I'm pretty smitten with my choice.
As far as the likelihood of a server going down, that's hard to say as you have no real way of knowing one will be more reliable than another at first glance. TBH I think the smaller servers in the network have the advantage here, their costs to run the server are lower since they have fewer users and less traffic. If they have the creation of new communities turned off, the space required is far less than otherwise. The main detractor with tiny servers is easily discovering whats on other servers. That's a longer explanation why that is, but it's like that and I've gone on too long already.