I voted scanlines. Scandoubled screens may be brighter (on VGA - RGB non-interlace with true scanlines has no brightness problem), but they look like blocky crap. Just take a look at a sphere or curved line. The blank lines help with the illusion of roundness. If you double the lines in between (thereby altering the original display), a sphere becomes a jagged square and a curve becomes a jagged line.
I'm not too thrilled about how scanlines are implemented in WinUAE though - especially because they're just black lines and if you switch quickly to an interlaced screen they're still there and moving the pointer removes the scanlines one by one (?!).
Now that most emulators have reached near-perfect emulation of various systems, I think the most important emulation improvement in the future will be "display emulation". Emulation can never be perfect until you can reproduce the same glow & scanlines (even flicker) of original displays.