Abrac
The main difference between purple and green is the size. Here are a couple of links that might help.
Comparison Table
http://www.tarotgarden.com/library/decks/thothtable.php
Old thread, List of Thoth Editions
http://tarotforum.net/showthread.php?t=35594&highlight=list+thoth
It may sound a little extreme, but these days I wouldn't buy a Thoth unless I held it in my hands and looked at the cards. You might be able to find one on eBay that has good pictures, that's another possibility. Avoid sellers who use stock photos. So many different versions and boxes that look alike, bait and switch as Barleywine mentioned; and as Le Fanu said, the older version with three magi gone now but the new box looks identical, minus the notice that said "Includes three versions of the Magus card."
Comparison Table
http://www.tarotgarden.com/library/decks/thothtable.php
Old thread, List of Thoth Editions
http://tarotforum.net/showthread.php?t=35594&highlight=list+thoth
It may sound a little extreme, but these days I wouldn't buy a Thoth unless I held it in my hands and looked at the cards. You might be able to find one on eBay that has good pictures, that's another possibility. Avoid sellers who use stock photos. So many different versions and boxes that look alike, bait and switch as Barleywine mentioned; and as Le Fanu said, the older version with three magi gone now but the new box looks identical, minus the notice that said "Includes three versions of the Magus card."