I prefer BOTH.
While playing old Amiga games on the TV they can sometimes look like they are being played in higher resolution than when using a scandoubler -> VGA solution (or whatever the like). Also, to me scanlines makes the lack of colours on OCS/ECS games sometimes become less apparent than when running them without scanlines (which also is one of the reasons I never really noticed how much less colours Monkey Island on the Amiga had to offer compared to any other 265-colour version of Monkey Island). Without scanlines, it just looks less colourful (but the screen becomes 'brighter', yes).
Though, I do like the crispness of the non scanline picture... it adds a certain feel, no matter how much more low-res and less colourful the picture might seem. Also, it makes it more interesting to compare a non-scanlined picture of an Amiga game with, for example, pictures of the DOS-equivalent.
In the end... I prefer both... but it completely depends on my mood. I will NEVER accept the point of view that scanlines never ever have anything good to offer... but, yes, while running the Amiga on anything but an ordinary television I do not often use scanlines... because as it already has been pointed out, there are other filters which emulates the TV-feel way much better.