Question about bore sighter

I emailed EZ Shoot to ask why they had two different sized sighters. Their answer was completely unhelpful ("There are only some differences in structure, function, price, size, etc. You can choose according to the size of the gun bullet and your own preferences"). Uh, ok guys.

Can someone with a little more knowledge explain why they have two 9mm cartridges? How do I know which one will fit my handgun?