The two slots are not really comparable
Zorro II is an expansion slot with all access to system addressing and I/O
and the ability to do DMA bus mastering and autoconfig.
The reason that RTG card used Zorro II (or III) is because they are generating their own video signals
The Video Slot is really design to access the amiga's RGB, digital video signals, clock, and colorburst. This really great for mixing amiga video with other video sources or for processing video or augmenting amiga video.
ie useful for genlocks, video toaster, flicker fixer. It is not as useful in the days of ATSC DTV, HDTV etc.
The closest thing to a video card that I can remember for the Video slot was the Opalvision.