I'm not a big partier but I lived in Southwest and it was fine. It got a bit noisy at night during the weekends but it was not like a huge deal. Sometimes there were incidences (like throwing up in the bathroom) but that's college in general. It's totally okay if you don't party, but you might be able to find people who are more low-key in the weekends in the other living areas.
If you give us your major it will be a bit more helpful because then I can get a sense of where your classes will be (and that will help a lot since depending where in Southwest you can be a bit aways from everything else on campus). Like for example, if you are a math/science major I wouldn't recommend SW because your classes will be farther. Yes there is a bus, but its always better to be closer to your classes than to rely on the PVTA. From what I have heard, OHill is a good community and has the largest rooms on campus. However, its up a very steep hill which be a pain in the ass (and I'm not kidding about the hill). During summer orientation you'll be toured around all of UMass's dorms and then you'll get to pick your housing.
As for diversity, I don't think you'll be in that much luck (UMass is majority white) but, the Latin American and African American community centers are in SW, the Native American center is in Central, and the Asian center is in NorthEast so if you consider having those centers close important to you, take that into account.
Let me know if you have anymore questions! I'm glad to help.