Last time I uploaded to galleries in 2020 I thought Rebrickable had a nicer presentation for MOCs than Studio Gallery. However the Gallery looks much closer to Rebrickable's MOC pages than I remember, with a picture album, likes, and comments.

Which do you prefer to upload to?

I prefer using the studio gallery, as it is easier for me to understand, and I prefer looking at digital models rather than downloading pdfs of instructions.

I know Rebrickable supports indicating that a model is built from parts from an existing set. Does Studio Gallery support that?

I do both to get more exposure for my projects. Gallery doesn't allow one to sell PDFs and you have to tinker with the design of the MOC to remove any steps created for instructions purpose. I don't mind my small creations being recreated by others, but for the bigger stuff that I put a lot of time and effort into creating them, I'd rather be compensated for the work done if someone else thinks my work is good enough that they want a copy of it. No matter the theme, I think all Lego work is art be it a building or vehicle to a mosaic.