Hello,
From what I have heard the Internal Scandoubler is better quality. External SDDs had Heat issues.
Using a ScanDoubler on a LCD Panel only work well if you are useing the LCD's recommended Resolution and Refresh rate, otherwise the picture looks worse than trying to use an Amiga Monitor.
Supposedly if the LCD Panel supports a NTSC/PAL Signal then the natuve Amiga modes will look significantly better. I've never tested this scenario, Can't afford the Samsung LCD's that have this feature.
Cheers
Shaf