Below include some of the most common Mystery Snails available in the trade.
Most are just labeled "Mystery Snail or Golden, Black, etc... Mystery Snail"
The Following diagrams are hand created on Microsoft's wonderful Paint program, so details are NOT perfect. To ensure closer accuracy, I Google'd the shell of each and compared at the end my drawings to them and decided to borrow the idea of angle diagrams from Applesnail.net.

Disclaimer
So, since these do look like cheap facsimiles of the (C) S. Ghesquiere Images posted on Applesnail.net, I will hearby give the before mentioned person comparison fact checking credit in the versions I hand drawn. In addition I also hearby grant open rights to anyone wishing to use these images I have drawn as public domain property.

However these, they do capture the essence of each specie's shell.

Snail canaliculata.bmpSnail paludosa.bmpSnail Diffusa.bmp
Pomacea Canaliculata_________Pomacea Paludosa_____Pomacea Difussa
or Pomacea Insularum

Pomacea Diffusa/Bridgesii*

Food: Likes to eat Algae, Fish Food, Algae Wafers, and Dead Plant Material. Won't eat healthy plants unless other food is not availible.
Size: Usually 2 1/2 Inches or 50-65 mm.
Breeding: Sexually, male and female pairing

Pomacea Canaliculata

Food: Likes to eat any and all plants in aquarium trade. Will eat fish food, but prefers fresh plants.
Size: Depends, can get to about 3 inches or 75 mm
Breeding: Sexually, male and female pairing

Pomacea Paludosa

Food: Fresh vegatation/plants
Size: Usually about 2 1/2 Inches or 50-65 mm
Breeding: Sexually, male and female pairing

Pomacea Insularum

Food: All kinds of Plants
Size: About 1 1/2 Inches or 40 mm
Breeding: Sexually, male and female pairing

The two most common to find: Pomacea Diffusa and Pomacea Canaliculata.
In conclusion, when choosing a Mystery Snail from a Petshop, you must look carefully and determine if the snail before you is a Diffusa, or anything else. If you have no plants or plans for planting the aquarium, your choices are not so limited. But if you enjoy the lush green of your aquarium and hate Algae, then Pomacea Diffusa is the snail of choice for you.


*Bridgesii is the old name for Diffusa