[Snowman]

SGC is by far your best option right now unless you're looking to grade modern cards that are worth ~$3000 or more, in which case you're probably better off going with PSA. You can grade with SGC for about $30 per card and with PSA for about $200. If you feel like you're on the fence over which one of these two to use, you might want to look up ebay sales of your card in both slabs to see what they sell for in similar grades.

As far as what grades to expect, PSA and SGC will generally give you very similar grades. The primary difference is with 9s and above. SGC has a 9 Mint, a 9.5 Mint+, a 10 Gem Mint, and a 10 Gold Label Pristine whereas PSA only has a 9 Mint and a 10 Gem Mint. So as you'd expect, some PSA 9s might get SGC 9.5s which often outsell a PSA 9. But these are subtle nuances that you probably shouldn't worry too much about until after you figure out how grading works and what grades to expect. It takes time and extreme attention to detail (and the right tools) to be able to identify what sort of flaws they're looking for. People who are new to grading are often very frustrated because they send in cards expecting 9s and 10s and they come back as 7s or 8s. Many flaws that cause a card to get a 9 instead of a 10 often can't be seen without 10x magnification.

Once you get all that stuff figured out, then it's also worth learning about the differences of how each company grades cards. PSA is much more strict on corners than BGS, and BGS is much more strict on centering than PSA. SGC is more closely aligned with PSA's grading standards (although just sliiiiightly more laxed on 10s in my experience) while CSG is more closely aligned with BGS standards.

Whatever you do though, I definitely would not send anything to HGA. They're a joke.