Are you checking bags? No bag fees on WN. That's a big one. You'll have to pay on US, I imagine. Also, on WN, I think you'll get a complimentary snack box on that flight. US will have Buy-On-Board options.
If seat-selection is important to you, that's a reason to choose US. You can select your seat ahead of time and not have to worry about whether someone ahead of you inline will take the set you want. With WN, you can pay for the "Early Check-in" which will automatically check you in 24 hours before hand so you can get a good number in the A Boarding Group.
US Airways offers 6 nonstops a day
Southwest only has 1 nonstop
Personally, unless flying US offers you an aircraft type you haven't flown before or has a flight at a time of day that is better for you, I'd take WN. While I've never had any bad experiences with US the times I've flown them, I just think overall WN is a better option.
"Let's Roll"- Todd Beamer, United Airlines Flight 93, Sept. 11, 2001