What is the difference between a 15 amp and a 20 amp GFCI ? Is it only describing the face plate or is it the amount of current it will handle ?

The amount of current it will handle, 5 more amps. In many cases, a 15 amp GFCI will be put on a #12 wire. Be very careful not to put a 20 amp GF on unless it is a #12 wire. The face plate only identifies it with the slotted neutral so you can tell the difference from a 15.

Also a 20 amp receptacle the slots will not look like 2 parallel slots, one will look like a T. Can you tell us what you are doing or why you ask?

Actually the only difference is the face plate. It has nothing do do with what it can handle.
A 15A GFI is rated for 20A feed through.

It has to do with what is allowed to be plugged in.