I've sailed twice each on Princess (Sea Princess to Alaska and Star Princess to Mexico) and Disney (Magic Western Caribbean and Panama Canal). I loved all of the cruises.
The balcony room on Princess was a good size but the inside cabin felt much smaller than on Disney. The food was very good on both. Princess had a bigger menu and to be honest, it was a lot easier picking something off the menu each night than on Disney. Another thing I liked much more about Princess was the accessibility of food throughout the day that was sorely lacking on Disney. We had late dining on both our Disney cruises and after the lunch buffets and restaurants closed (usually after we get back from port), we were stuck with that "theme park food" on deck 9. At least on Princess, we felt like we had no problem getting good food any time.
Princess doesn't come close to Disney with the entertainment. While I saw the shows each night, it was mainly cheezy Vegas-style cabaret shows. They were entertaining but not my thing.
The staff on Princess were great too. I had no complaints about the level of service at all.
I'm really sensitive to smoke too and it wasn't bad on my two Princess cruises. I did have problems on my balcony with the cabin just upwind from us but that can happen on any cruise. The smoke was a little thicker around the casino but elsewhere on the ship like near the pools wasn't bad.
Since Disney is primarily non-smoking, it felt thicker to me in certain areas like Deck 4, the adult pool/Cove Cafe, etc.
We love Disney the most but probably won't be sailing with them much anymore unless their prices go down. We'd love to try
Royal Caribbean but would jump at the chance to go on Princess again.
Whichever other lines you choose, be sure to check the reviews and ratings on the ships. Whereas Disney only has two ships and they are both kept in great shape, with the other lines, their ships very greatly in age and they can really show it. As much as we loved the Sea Princess, it was very different than the (then new) Star Princess.
-steve