As someone whp has been to both places many times I would say they are two totally different places.
Negril is famous for the 7 mile white sandy beach that is much more beautiful, relaxed and more upscale than Sousa. Often voted one fo the top 10 beaches in the world. Negril is the #1 place on the island where the more well to do Jamaicans vacation. Sousa town, which is very close to the beach is considered the red light district of the D.R., alot of prostitutes and all the negative that brings, which is visible in the day or night...a more seedier area of the island. Negril in my opinion is much more fun, you will find better service and quality, you are bothered less on the beach, easier to mingle with the locals who are NOT looking to financially benefit from you, people there are more hospitable, a lot of beach parties, bars, good music, and a better nightlife.
I prefer Negril hands down.
It depends what you are looking for.