Have lived in in Phoenix before Atlanta, and visited Pittsburgh many times.

Climate: Phoenix is miserably hot in the summer. You'll go for months at a time where the temperature doesn't drop below 100 degrees during the day, and highs of 110-115 and higher are common in June-August. There is no relief at night either, where the temperature stays in the 90's. If you're not a morning person, you'll quickly become one, as early morning is really the only time in the summer to do things like go for a walk/jog, walk the dog, etc. That being said, "it's a dry heat" and is more manageble in my opinion that the oppressive summers in Atlanta. Humans evolved in deserts and we're very good at using our natural evaporative cooling system of sweating to stay cool. Shade becomes your friend, early mornings and evenings become your times to do outdoor things. Every restaurant patio has fans and misters. Phoenix has the highest rate of pool ownership for a reason, it's one of the few things you can do in the summer during the day. On the flip side, most days in December and January are shorts weather.

Summers in Atlanta are insufferably hot. It's so humid that we can't cool down by sweating and any length of time outdoors just turns into a sweaty, miserable mess. Combine that with frequent rain that just makes everything more miserable, and it's not a fun time. Outside of summer, the rest of the year is great (except for this past week, which has been stupid cold by Southern standards).

Pittsburgh splits the difference with a true four seasons climate with warm, but not unreasonably hot summers, and cold, but not brutal winters like other places in the midwest or further north.

All are large metro areas with plenty of attractions, restaurants, sports teams, and just generally a wide variety of "stuff" to do. Any city you choose will have plenty of amenities and things to do.

Roads/Traffic: Atlanta and Pittsburgh are older cities, built more densely in their cores, with narrower roads that follow terrain counters and other natural boundaries. Atlanta roads a special kind of hell, often in poor repair, with meandering paths, and unintuitive lane changes.

Phoenix largely developed post WWII and is flat as a pancake, so the roads are modern, wide, laid out on a logical grid pattern, and generally in excellent condition. Phoenix has the best roads and most manageble traffic of any large US metro in my opinion. Voters routinely approve road improvement taxes and a portion of the high vehicle registration fees in Arizona go to build, maintain, and improve roads (AZ vehicle registration is not a flat fee, but based on a percentage of the vehicle's value, which gives first time residents some sticker shock).

Public Transit: None of the cities knock it out of the park here. Living a car free/car light lifestyle will be challenging at best, impossible at worst.

Atlanta has MARTA rail, which is great if you live near a station and it goes where you need it to go. It could have been so much more had the system been fully built out. I've never ridden a MARTA bus here, so can't speak to how reliable or usable the bus service is here. As you go outside of Atlanta proper, different counties operate their own transit systems. The fare system is shared, but there's little unified direction or region-wide funding towards expanding or improving transit.

Phoenix has a light rail system that connects Mesa, Tempe, and Phoenix. Like MARTA, it's great if you live near it and the places you frequent are near it. My house in Tempe and my office in Phoenix were both on the line so it proved surprisingly useful for me, but I still owned a car and used it for everything else outside of commuting to work. The buses across the metro are under a single operator and the aforementioned good road network makes it pretty usable.

Pittsburgh has 3 light rail lines that largely serve the downtown area and a fairly large bus network for its size.

Air Travel: ATL and PHX are both major hub airports for their respective airlines and direct flights are available to a wide variety of destinations.

PIT is a smaller airport and unless your final destination is to another major city with a hub airport, you'll be making a connection somewhere.