Sessions were great but on the other hand using a Speed Dial folder (or a bookmarks bar folder though a command to add all open pages missing) increases the flexibility significantly. You can add/move/remove pages from the saved sessions with drag and drop so managing them is much easier.
You can add specific pages manually to Stash too, but it's a bit more search-driven storage than sessions, as the purpose of it is to index pages' content so you can filter the pile and show huge screenshots for recognition.