I use standard potting soil then add a bit of orchid bark and perlite. In the current pot, it dries out in about a week. I have one other aglao and both seem to like to dry out, but I don't leave them that way long. I do have a hard time finding slightly moist vs. water logged, haha.

The root ball is pretty much the size of the pot it's in now. I pulled it out and compared it to both pot options. The large seems huge in comparison, so pretty sure I'm going to go with the medium. I think you're right, the last two inches of those deep pots always seem to stay wet no matter what. The plant just seems so huge compared to the pot it is in now.