When you grade a card, you become a stakeholder in the perception of the grading company.
The Canadian grading companies are a waste of money because nobody in the hobby values them. Adding an MNT or KSA holder to your card generally does not add value compared to the cost. To verify this, go to eBay and type in the term "MNT 9.5" young guns. You can verify that MNT 9.5's generally sell at the same price as raw Young Guns, but then deduct the grading costs that the owner originally paid, and the card was actually devalued.
For vintage (Gretzky), PSA is the flagship. If you are grading it to sell, find the most economical way possible to get it into a PSA holder.
For modern hockey like Young Guns, BGS has been the flagship for many years. PSA is becoming more acceptable because of numerous factors. I think part of it was the speculators coming in from other sports during the pandemic. Other sports view PSA as the flagship grading company.
I also think that a lot of novice collectors don't look at grading rationally. They view it similarly to opening boxes. It's gambling. They don't own a loupe, they don't pre-grade their cards, look at comparable sale prices, and then make a calculated decision about what they should send in and whether the odds of a higher grade justify the submission price. Go to a site like Reddit that is loaded with novice collectors and see every "should i grade this guys?" post. For these people, PSA is the better value proposition because the bar for a PSA 9 or 10 is lower than the bar for a Beckett 9.5 or 10. So if they send things in blindly, not only is it cheaper per card, but the outcome is more exciting.
The best way to sell a graded card is through whatever marketplace offers you the most customers and the best rate. eBay is still the top market place for cards, unless you get lucky with a local sale that has no fees. If you list locally, odds are you will get lowball offers from resellers. I am not familiar with the cosigners who operate on eBay and who is trust worthy or easy to deal with, but they do advertise fee rates that are lower than what you would typically pay on eBay. I would not sell a four or five figure card on eBay on your own if you aren't experienced with the platform.