From an investment standpoint what would be the best convertible to buy?
From an enjoyment perspective, Maseratis are positively amazing. From an investment standpoint, these cars may be the worst financial decision you ever make. Buy a Maserati, drive it as often as you can, and enjoy everything it offers. If you start thinking about money, all the fun goes away very, very fast.
Any special edition convertibles like the Shelby Edition Vipers?
In 2004, Maserati released a special edition Spyder as a 2005MY car. The car was called a 90th Anniversary Spyder and is one of the rarest modern Maseratis due to the low production volume (90 made). Here is Maserati's official page on the car...
Maserati Heritage - Maserati Spyder 90th Anniversary
You will have to decide for yourself if the price delta, if any, between this car and a standard Gransport Spyder is worth it.