-Location/environment/weather. Seattle will be gray and rainy (but not very cold - chilly, not cold) throughout the majority of the academic year. This is not objectively a bad thing, but it is what it is. Chapel Hill will be warmer and probably have more sunny days. In September and October and April and May, Seattle will (if it decides to) be warmer and sunny more often. Chapel Hill may be uncomfortably hot, particularly in May and September (and August definitely, if you happen to be there then) but everything is air-conditioned anyway.
Also, UW is in the middle of Seattle, a large city. UNC is in a small suburban college town (Raleigh is pretty close - about 40-50 minutes - but I’m not sure how much that’ll mean if you don’t have a car.)
-Your potential peers, but they will be relatively similar at UW-Seattle and UNC-Chapel Hill. (UW will have a little more of a mix of high school academic achievement, whereas UNC is so competitive that they have a greater proportion of top students.) They have similar proportions of in-state students (a little more than 80%).
-The kinds of majors and classes you can take at each college and what appeals to you academically. That’s not necessarily major “strength,” as you’re not going to study a specific “program” as if you were a grad student, and you may change your mind. I would only use that as a tiebreaker. (For example, UW has great computer sciences and related majors, like informatics and human-centered design and engineering. UNC has excellent public health and business majors. But both are great comprehensive universities.) I’m talking about the breadth of things that are interesting to you at the school that you could take; or if you wanted to study specific languages; or study abroad in a specific place, that kind of thing.
Both universities have cool majors in areas the other doesn’t: UNC has a variety of things in the health sciences like dental hygiene, clinical laboratory science, radiologic sciences, and public health. UW has a bunch of majors in tech and computer science, like industrial design, informatics, interaction design, and the HCDE program I referenced above. At UNC you can study languages like Swahili, Wolof, and Lingala; at UW you can take languages like Danish, Finnish, and Norwegian. And so on. You can look at the major offerings at both universities - as well as minors, special programs, centers, study abroad programs, etc. - to decide for yourself.
-Additionally, think about the extracurricular things you’d like to do as well. Are there certain clubs, organizations, intramural sports, etc., that you’d like to do or that seem interesting to you?
If your family faced an emergency financial problem, will they be able to pay OOS tuition costs? These schools are both expensive for OOS students with no aid. Did you apply to your own state’s public universities?