I believe it will be US Airways, because
1) the plane fits the route system so well, it is almost like the plane was designed for them specifically.
2) the plane is available now, and only takes 10 days to change a DC9/MD80 pilot to a 717 pilot. Maybe less.
3) the 318 which is also being considered is too heavy to do the short routes that US would want to do with a 100-pax plane.
4) Boeing, desperate for 717 buyers will buy all of US' DC9s and F100, and perhaps 732s in trade for the 717.
Now before you go and say that US has committed to being all Airbus, think again. That simply isn't true. For now at least, Boeing has the midsize jets in the fleet with the 757/762 that US is not removing. US has signed no exclusivity agreement of any kind.