I am also doing Sociolinguistics, and just went through the application process this year. If your interests in socio are mostly in variation studies (as it sounds), I would recommend University of Michigan, University of Pennsylvania, University of Toronto, Stanford, UC Santa Barbara, and maybe a few others.
It is true that some of the biggest factors are your statement of purpose and writing sample. Some other things to consider are how well your interests compliment potential advisors and other students at a particular program as well - a program that only has one professor that you work with is not going to be as strong a fit as a program that has several people with whom you can collaborate and work with.
Just a few things to consider. If you're planning to apply next year, you still have time to work on other research, visit schools that interest you, etc. I also agree that finding articles that interest you and contacting the professors and schools where they came from is also going to be helpful for honing in on your interests and directions of study.
Good luck!
