Scanlines look all right on LCDs, because they have an insane overcontrast. I recommend 1920x1200 resolution, fill screen, and set scanline transparency. You don't need pitch black scanlines, you know.
Looked GREAT on my 1920x1200 15" laptop screen, not so good on my $1200 27" 1920-LCD.
On CRTs, contrast is not "consumer-adapted" like on LCD, so enabling scanlines usually looks drab / dark. Maybe I'll try 1920x1200 on my CRT, but...
Native resolution and Hz is always best, ie. the glorious 1084 etc. \o/ My preference for WinUAE is 512x384@50 Hz on my 21" Dell Trinitron CRT, "Lores/Normal" in Display setting. Nice centered 14"-size picture in native resolution, no scanlines needed
I'm talking game and demo perfection in emus now.
If you spend your time in Workbench, just go with 060+AGA and highest resolution and go windowed and don't use scanlines.