I'm currently thinking about offering both buttons, and the usual full hotkey configuration for them in the options. I feel the love for both buttons myself, even though it bloats the current end-of-game dialog.
Next level should be disabled iff the bottom level of a directory has been played.
Next unsolved level should be disabled iff all levels in a directory are solved. It's not necessarily disabled when all levels below the current one are solved, this button will wrap back to the beginning of the directory.
Maybe it's not going into the very next release, but
next-level button is a short-term goal in the Lix wishlist. I'm planning to make a physics-bugfix release first with the
soft time limit in singleplayer, because that affects physics, and it's yet to be implemented.
(New game versions with physics changes require everyone to update in order to play multiplayer. Without physics changes, people can have whatever version they like. I strive to condense all physics changes into a single release.)
Proxima: I'm planning to have the next-level button in the end-of-game singleplayer menu. Since you mention clicking them repeatedly, do you want them on the main game screen? That one has run out of space in the panel as I see it...
-- Simon