If you're on Reddit by any chance, this is the question that gets asked about a billion times per millennia... And yes, 1000 years ago reddit didn't exist, but I have a feeling that in the year 3000, that question will be in reddit's archives one billion times, so just so that no one can say, well, I didn't find it on WFA blog, so I'll go ask on Reddit, here's a consensus:

Should I become a Union Actor right now?

Generally you want to wait until you have a good reel, plenty of experience, and an agent you trust/like before trying to become SAG  Basically once you're in the union, it means you'll be able to audition for better paying gigs, studio films and television shows, but it would be your agent that will actually be the one getting you the auditions. You just gotta do the rest.  A lot of people think this is great, however, when you're SAG, it's very likely that you're working less and you're competing against actors at a higher level.

If you're in a union, you can't do non-union work. Did I say "can't"? I meant CAN'T. Or at least you're not supposed to. 

What does it mean to be SAG eligible?

It means you've done work that makes you eligible to join SAG. There are few ways to get that. Usually this is done through getting a speaking role on a union project or by getting enough SAG vouchers through doing background work. 

So what about staying Non-union?

Low-budget/no-budget films can't afford SAG actors, so SAG actors aren't allowed to work on those films and all the non-union actors get to have a pick of the litter. There's definitely more of those jobs out there, many times you might end up with someone who's not professional, those are the risks, but you also compete at a lower caliber and have better chance of landing something great.

Consensus once more:

Let me put it that way... if you're asking yourself "am I ready to become union?" then you're probably not ready. You are ready if you've given it a lot of thought and understand exactly what it means and are 100% ready for it, give it a go. But if all you've done thus far is read my paraphrasing of hundreds of reddit responses, then please, give it some more time before you pull that trigger.