I understand you are worried that there isn't much of a difference between the two because they are bright but... have you actually checked?
If you are alone and don't have a buddy to help set your phone on something around the same height as your truck and have it record a video. Then just go turn on your lights and confirm your suspicions or not.
I don't know what type of LED tails you have but I have had an ebay set and now I am running the 13 LED tails and in both there was a noticeable difference between parking lights and brakes even during the day.
This is what my old ebay set looked like during the day with just parking lights. Bright sure but there is a massive difference between this and brakes.