It really depends what you will use your Amiga for as both cards have their advantages.
The Blizzard Vision is a very powerful RTG card so this will give you a nice 24bit Workbench up to 1600x1200 and Warp3D for games like Quake, Wipeout & ShogoMAD.
However the main drawback is you still need a second connection for your AGA stuff so many people use a monitor switch box or in my case I have an LCD TV so hook up the AGA via RGB Scart and switch from PC to AV on the TV
The Indivision AGA isn't a Graphics Card, its a Flicker Fixer and Scan Doubler, it displays all the modes AGA and HighGFX (up to 1024x768 256col) on your Monitor so there is no manual switching involved. This is the best option IMHO for a non PPC System.
You can also have both in the same machine.
Steve.