Choosing The Right Pick

The biggest contribution a gemstone pick, or any pick, makes on a stringed instrument lies in the shape of the tip, due to how it comes off the strings, and the thickness and material of the tip.
The thickness of the center of the pick has no bearing at all on the string vibration.
That is why you can buy guitar picks that have indents in the center and there are even picks that have a hole in the center of them. It is there for the comfort of the player and to help them hold onto the pick better.

Again, it is only how the tip of the pick reacts to the string pressure that makes the thicknesses important.
Rhythm players (those that just strum their guitars, mandolins, ukulele and others) usually prefer a rounded tip and people that like to play a lot of single line notes or those that play electric guitars prefer the sharper tip.
Guitar people are very particular about their pick thickness and we try to have answers for their needs.

We used to sell picks that had three different shaped tips. The idea was that the player could choose which tip to use when playing. This sounds interesting but it is not practical and it is not popular. Guitar players will often turn the pick in their hands because it slips during play and it would be impossible to know which tip was exposed to the strings. Turning the pick in your hand is also a nervous little twitch that most players experience.
Up until the past twenty years or so the most widely pick used was the traditional shape or sometimes called Classic pick.