Luggage concern: Airbnb vs. Hotels?

I have fairly often stored luggage at a bus or train station, though never in the cities you are interested in. There has never been any problem. I do use a small lock on my suitcase to make it just a bit of a challenge for any potential thief, but I've never heard that theft from checked bags is an issue.

I suggest looking in your guidebook for info and also Googling "luggage storage" (and name of railroad station you will be using). Then read the most recent links that pop up to see whether there are any particular concerns. The likeliest issue would be the need to allow plenty of time if you are using a staffed counter rather than a locker. There can sometimes be a wait to retrieve luggage from a staffed counter.

I have stayed in airbnb apartments in several cities in Europe, and I have usually been able to make arrangements with the owners to store my luggage until later in the day if I had a late departure. This is not always possible, but you can always contact the host prior to booking and explain your itinerary and see if they can offer to let you drop off your luggage early or store it later.

I use Airbnb a lot now. Most are pretty good about allowing you to leave your bags. It will depend of course...if it's an apartment and they have other guests coming that day (or guests there the morning of your arrival), it's understandable that they wouldn't want you coming back when other people are there. If you go with a room in a house/apartment, they may have another area for you to leave it so as not to interfere with the next guests.

I'd ask before booking. If it's a deal breaker, look for someone who does have a spot you can leave it. We've also...at times...left our stuff at train stations and in lockers with no issue. And of course, most hotels are pretty good about letting you leave your bags.

Here is basic info on storage at Paris stations and at CDG airport. If you're arriving by train at noon and leaving by air that same evening, I'd say store it at Gare du Nord if you'll be using the RER to get to CDG. If arriving at a different station, get it to GDN before you start your brief sightseeing. If you'll be taking a taxi to the airport, then store it at the station where you arrive if it has storage.

You didn't ask, but if you're actually spending just an afternoon in Paris you'll be very pressed for time. Getting to CDG or Orly will take some time and you need to be there early to check in. Leaves little margin for error.

I've never asked an airbnb host to store luggage, but I haven't used that system much and I guess it's worth asking. Should be no problem with a hotel.

There is luggage storage at the Santa Maria Novella train station in Florence. I used it in 2012 while traveling from Rome to Venice by train. If you are arriving by train, it is quite easy to use and you can drop your luggage off and explore the city. There is also luggage storage at the Venice train stations. Storing luggage until your apartment is ready should not be an issue with your itinerary.