Hi there, I think I can offer a little bit of explanation of the two.
Heelside progressive edge backroll: Ok, this is the typical backroll. The trick is done by continually edging harder and harder as you go up the wake. You edge up and away from the boat at the top of the wake and push your hips and chest up to the sky. You will do almost a perfect cartwheel rotation and some people will actually pull to revert then whip back at the last minute. The backroll done this way is good to know for roll to reverts or heelside mobius tricks, it is also sorta handy for learning the edge for raleys (my opinion).
Heelside mexican backroll: Same trick, just done differently. For this trick you set your edge early, then flatten off right before the wake. You initiate the roll not by your edge, but by throwing your chest down to your toes. Usually, people consider the mexican backroll to be "incorrect" because you are not edging all the way through the wake. The rotation of the mexican backroll looks more like a front flip, which is why it is often incorrectly called a frontroll, which doesn't exist. It is harder to spot the water on the mexican backroll because your chest is turned down towards your feet instead of facing the boat. The mexican backroll is a good trick to know for the roll to blind and KGB tricks. It is also generally easier to grab a mexican backroll because you are usually in a tuck position and can go for an indy grab. Most new riders that I see will throw a backroll this way because they think they have to throw the trick and do not edge properly.
Does that clear it up? Both are backrolls, but they look different and are thrown different.