Just to be clear:
Indivision doesn't do anything to speed up the native Amiga output, it just converts it to be VGA monitor/LCD compatible. But it works this way with anything you throw at it - games, demos etc. It also allows you to use some high res graphic modes in WB like 1024x768, but still in AGA speed.
A graphics card accelerates the output by generating the image on its own chip, but in doing so it becomes incompatible with vast majority of games and demos and so is only effectively usable with WB and OS-friendly apps.
So, as others have stated, it's best to have a combination of both - some form of SD/FF (like Indivision) to play games and watch demos, and a graphics card to make WB faster.
Picasso IV and Cybervision 64/3D with SD/FF module take care of both at the same time - scandouble native output and accelerate WB.
