THAT is the toughest question of them all!!! It’s ALL a compromise. All of my spectro systems are 12mm (and now a 16mm) to get a larger image scale but that does sacrifice a lot of sky with the narrow fov’s. I did a short test with a 3-8.5mm f1.0 lens set to ~7mm a while back and that gave a decent result spectroscopically so I’m planning on running that a bit longer after the summer when the sky gets darker again. There’s also the limiting mag issue, that’s another set of problems…
I’m tempted to say 12mm is the best just from experience but there are a lot of variables! No easy answer I’m afraid.