Dan, I don't have experiece with either model, but I do have a used Ryobi 16-32" model that at least qualifies me to talk about drum sanders.
I can't think of one flatwork project I've made over the past 5 years that didn't involve the use of the drum sander. I have a jointer and planer, but the drum sander helps with final prep.
Between the two you list, the first question I'd consider is what would most often be the widest piece of stock you'd run through? The 10-20 gives you the option of over 10", whereas the Grizzly limits you to 12". That's important if you're making cabinet doors, table tops, or resawn and bookmatched panels. It's not so important for veneering or other smaller widths.
They generate lots of dust, so keep that in mind in your selection. The Grizzly only has a 2-1/2" port. A drum sander doesn't generate a lot of noise, but the vac. attached to it does.
Have you considered the
Grizzly G076? It's 10", but if you found the need to run something through wider than 10" you can probably remove the lock knob. Newer versions of the 16-32 drum sander similar to what I own have the same style of lock knob. The G076 is less expensive than either model that you referenced.
If I was spending $764 on a drum sander, I'd spend $150 more for the 18" from Grizzly, or work on finding a local craigslist listing for something less expensive and bigger.
Good luck in your hunt, and I'd be happy to expand on my experiences. I think a drum sander is great tool to have. Don't mistake it for a thickness planer, but with patience you can flatten unruly grained wood.