A Mexican Backroll is just another version of the same invert. Both are backrolls and both serve a specific purpose.
A regular backroll lets say heelside, consists of popping off the wake and sending the tip of your board up and back away from the boat also turning your head over your back shoulder will help the rotation to get around. Eventually you will come around and land square. This type of backroll is used to learn roll to revert and heelside mobes.
A Mexican backroll is the same move just thrown a little different. Instead of throwing the tip and your head around away from the boat you simply throw your head down and kick your legs up over your head. This is exactly like doing a front flip on the trampoline. This type of backroll helps to set up roll to blind and KGB.
To me the mexican backroll is easier because it is a more natural way to throw it. You can practice it on a tramp anywhere. One suggestion though the mexican backroll sometimes has a tendency to hit the rope on the way down and you can land in a lipslide position sometimes because it isnt coming around like a regular backroll so just huck it a few times and get the feel of it.
Hope this helps to explain it a little.