Welcome to cruise-chat!
I agree with your assessment of Carnival. I've sailed many lines, including Carnival, Celebrity, Princess, and HAL, and have enjoyed every one of them. In fact, there was one year in which I sailed Carnival and Celebrity a few months apart and felt the Carnival cruise was superior in many ways. In some respects, it's the luck of the draw. In my years of reading and writing reviews, I've found that people are more likely to recount negative experiences than positive ones, so take these with a grain of salt, as they say.
In my opinion, you can't go wrong with either. (Yeah, I know that's not much help...) Then again, there are differences between ships within the same line, so the ship itself plays a role, as well.
The main differences between a Southern and Western Caribbean cruise, as you might have discovered in your research, are embarkation port and port-intensiveness. W. Caribbean cruises tend to have more sea days, and a wider variety of embarkation ports (New Orleans, Galveston, Mobile, and several in Florida), while a typical 7-night S. Caribbean cruise leaves from San Juan and is port-intensive. As to which is better--I can't really say, as I've enjoyed them both. Depends on what you want to do in port.