It's hard to say what exactly restaurants use unless you see the process. When I make them at home I take a 1 inch pork loin chop, butterfly it, then pound it out before breading and frying. While this is close it's not the tenderloin. I tend to think most places choose the loin over the tenderloin, its cheaper and easier to work with, and once you bread it and fry it, the difference in negligible unless you leave the loin thicker like West Town Pub does.
This is from West Town Pub's menu. Listed as a tenderloin sandwich but the description says they use the loin.
BREADED PORK TENDERLOIN SANDWICH
You can’t have a respectable menu without this baby on it. After all, we are in Iowa. A generous hand breaded pork loin served on a Kaiser roll with lettuce, tomato and onion.