I've never been to Killington, but I go to Sugarloaf a lot. The reason for the confusion about terrain size is that for total acreage, Sugarloaf allows skiing anywhere within their boundaries and so lists their entire mountain in their total size. But even now, when essentially everything is open, the available developed trail acreage is 485.
It can be cold, but this is Maine, right? The problem on some days is the wind, which can sometimes close the upper lifts. The upper mountain is still often accessible via a surface rope tow -- but who wants to go up there when winds are 80mph at the summit? Having said that, I wouldn't let weather keep you from coming up -- if you dress for the conditions, temps/winds are rarely a problem.
From what I understand about Killington, I don't think Sugarloaf would compare with respect to nightlife. On mountain and area restaurants and bars are there, and sometimes there are concerts at the base lodge on the weekend, but don't come up expecting a lot happening at night. On mountain lodging is good with lots of options for ski in/out. Their website has packages that include lift tickets and lessons and you can usually get pretty good deals. There are off-mountain hotels/B&B that can be cheaper. Check out their lodging page
Hope this helps. It really is a great mountain.