There's also an Avis location in downtown Pittsburgh, in the building I used to work in (reasonable walking distance from the Westin). If that appeals to you more than Enterprise, you could check with them. It's a franchise location, but they were always friendly with us. If you want to check rates, you're looking for the location at 625 Stanwix Street.

There's no Hertz location downtown. There may still be a Budget, but it's not quite as convenient to get to. I don't think anyone else has a downtown location. I thought the Enterprise was close, but I just looked it up, and it's not really. They'll probably make you ride back to the office with them, which could actually make Avis more convenient.

Since Nov 3 is a Thursday, you shouldn't have any problem with pickup/dropoff times. I still park in the same garage where Avis keeps their cars. (Yet another point in Avis' favor: the garage location keeps downtown street driving to a minimum.) They allow for after-hours drop, so even though I think they close at 6ish, you should be able to drop it that evening while incurring just a one-day rental and no unnecessary parking charges.

I don't think you should go for a larger car size at all. Go for economy and hope for no upgrade! On the off chance that it actually does snow in early November (not too likely, but it could happen) you'll be better off with a car size that's closer to the one you drive. Having a midsize vs compact or subcompact really won't make a lot of difference in snow performance. Not having to worry about the car being too big will be a much greater help. The cars will have all-season tires on I'm sure. They should be fine for any snow that would happen in early Nov.

Another thing that comes to mind logistically: if you're doing this drive on Thurs then leaving on Friday, then you should definitely consider staying elsewhere on Thursday night. Coming all the way back downtown is fairly counterproductive if the main thing you're doing Friday is going to the airport. Transport from the airport is a $30 or so cab ride or a $20-ish shuttle I suspect. (There may or may not still be a nasty uncomfortable couple dollar public bus route, wouldn't recommend except for the cheapest of cheapskates.) If your schedule works that way, then I would recommend getting the transport to the airport on Thurs and renting the car there, assuming the car rental is cheaper there. If it's not, then find out the cost of renting downtown and returning to the airport. Enterprise will probably work better for this since it's all company owned. The downtown Avis is a franchisee, but you can check with them anyway and see what they say. Then you can stay out at the airport that night. The Hyatt is decent. If you're stuck on Starwood, I think there's only the Four Points. Or you could stay down at Nemacolin near Fallingwater (probably pricey). You could keep the car all night, or if you stay near the airport just return it and use the hotel shuttle.

Of course, that big paragraph is moot if you have business downtown on Friday, but it doesn't look like it. If your flight is not first thing in the morning, you may be wishing to keep the car longer. Maybe just plot the 24 hour rental period accordingly (9-9, 10-10, whatever). If your flight is well into the afternoon, make it a 2-day rental.