I'm not a Latinist either.
If I recall correctly, the two inscriptions are believed to have been set up roughly at the same time. The second one seems to be even sloppier than the first, even though the lines are straighter. The upper lines are very crowded whereas there is an abundance of space at the bottom.
Then there is the question of spelling differences between the two inscriptions. I don't know enough Latin to tell which one would be the original and which one the copy, but since they can't seem to be dated relative to each other, I'm not sure anyone has got that figured out yet. It seems to me that late Romans had a much more 'dynamic' or relaxed approach to grammar and spelling. Languages change over time, so that might not necessarily be a sign of a decline. I hope someone more knowledgeable can chime in though.