If you run it under a 32bit OS, it'll download 32bit win8 image,
run on a 64bit OS, it'll download a 64bit image. That's the only difference AFAIK
Also running the assistant under windows 8 will fail with "not available in your country"
unless you run it in compatibility mode - well it did for me.
No need to download the ISO multiple times though - I've bought lots of keys at £25 each
- what I did was run the assitant multiple times to get keys (cancelling the download).
The first time I downloaded the ISO, and copied it to a USB pendrive.
Then boot/ install from that pendrive using the keys above on each PC