All of the above, mate. Yes.
I used U-Circular Silk Satin Heavy.
I think the only possible downside of the heavier weight I could mention is that it’ll have more of a tendency to lift where it drapes over edges. I trimmed off as much of the drape as possible before vacuum-bagging it… …oh, and you’ll definitely want to vacuum-bag it!
Its weight definitely helps it maintain its form better. Your design is less likely to get skewed. If you’re laying-up the fabric onto a fresh wet base, as I do, you’ve got plenty of time to get the positioning just right anyway plus the wet resin provides all the lubrication you need. Now that’s not to say I got it spot-on but it passes everything but the more forensic examination.
I think the only case I can see for a lighter fabric is if you want to show the underlying wood grain or whatever or if you can’t be arsed to vacuum-bag it. Otherwise, for my money, it’s heavy every time.
Edit: Just checked that Spoonflower Satin. The Satin Heavy is nearly 2.5 times heavier! (172 gsm vs. 75 gsm)