It's all about investment vs reward. SGC is the best value of the big three (BGS, PSA, SGC), though HGA is coming on strong and would be further ahead if the didn't have that Wayne Gretzky snafu. But, SGC cards will sell less than the other two.
Now, the paradigm on that is starting to shift but it's going to take a while. And SGC is the only one that has a lower cost service that's actually returning cards in a decent time frame. It's roughly $45 per card with shipping for SGCs value service and you'll have your card back in around 30 days.
It's around $120 per card with shipping for PSAs cheapest available service and only God knows when you'll get your card back. BGS is close to the same. There are people who have been waiting a year for their PSA value stuff back. However, when your stuff comes back from PSA/BGS, the same card, same grade will be worth up to 30% more than your SGC submission. Obviously you can pay more money for faster services (and actually have to pay more the more valuable the card is), but it comes down to a risk vs reward proposition.
A good rule of thumb is if you are going to keep it, don't grade it, but if you want it graded, pick the service that fits your taste. Each slab from each company has a very unique look, HGA even allows you to customize your slab. If you are grading to sell. Research what comparable cards are selling for at the grade you believe your card will achieve. If the cost to acquire the card plus the cost to grade based upon whichever grader and service you pick exceeds what you see, then it's a non starter. I actually like to plan for a grade below what I'm hoping for. Meaning if I think something is going to grade a 9, I look what 8s from the company I'm thinking about using go for and make my decision off of that.
You also have to be careful because if you send in a card that has a particular value and it grades higher than you expect, they will charge you the price of the value it graded for. So if you sent in a card and pay for the $1-$1499 value range and it grades a 10 and is now worth $10,000, they will charge you the additional amount for grading a card worth $10,000. I know it's complicated, but grading is a very good investment when done correctly. Too often people pay to have a card graded and the end result wasn't worth it. I've done it myself. I have a 1987 Donruss Barry Bonds that graded an 8.5 with SGC, so I paid $45 to have a card that won't even sell for $20. Happens to the best of us.