"Which one is more expensive?" - Impossible to answer it could vary by product but in my experience the airline duty free is often more expensive.
I have never found duty free from anywhere that I though was worth the bother of carrying heavy bottles around with me and I don't smoke which is about the only product that varies significantly from the prices outside.
Even with tobacco the limits are such that you only save a finite amount. For me "Duty Free" shops in airports are just places for captive bored people to pass the time between flights.
The risks of it getting taken off you by some security person at screening also makes me wonder why people still bother.
At least if you buy on the plane you only have to carry it to your taxi or whatever and you normally get FF points from the airline.
Most of the worlds DF stores and airlines have their price lists online so you can check prices for yourself.
The only problem with buying onboard the aircraft is that they have very limited selections and very little stock so they may (and often do) run out of what you want or just don't carry it.
LOL. What you are asking for is a comparison between every product for sale on every airline and duty free shop!!! And then what about your destination? It may be cheaper there than in any duty free.
All perfumes are cheaper in the airport and all ciggies more expensive in the air. Lol.
Seriously.., what a wide question:-))
In many cases, these "duty free" items aren't necessarily any cheaper than shops proper on the ground. You have to do your homework and know the prices.
Generally speaking, airline prices are cheaper than in airports, but a lot of this depends on where you are based, your currency and what you buy. Most duty free shops are starting to put their catalogues and prices online line now, but not all and some are introducing pre-order for delivery on arrival.
The only way to properly compare prices is to look directly at the duty free catalogue for your carrier, airport and destination... some sites are offering price comparison, but in my experience they are often inaccurate. There are so many rules related to duty-free shopping, the sizes of the packs or bottles vary and in Europe what is sold for EU travellers is not duty free at all! So, price comparison is almost impossible to do unless you just look at them for yourself and decide what you want to buy and where.
After many years of border-crossing and international travel, we have concluded that "duty-free" does not mean "profit-free".
While savings can be realized on alcohol and tobacco products, the spread seems to have shrunk to the point where the difference is often not worth the hassle.
As for cosmetics, there seems to be very little difference between the "duty-free" price and the cost of the same items in a regular department store.
I mostly buy cosmetics and alcohol duty free. For cosmetics, the internet has negated the allure of duty free and I can get something shipped free to my door after purchasing it cheaply on a website (eg strawberrynet.com), but will buy duty free as I'm relaxed on holiday and it's something I need to replenish. Alchohol is so heavy and annoying to carry around, so I usually just get it at my last destination, especially as the liquids restrictions mean it might be seized before that anyway.
Don't forget - if you buy in a foreign currency, you may have exchange fees and bank fees. I don't find Duty Free to be that much of a bargain once you take everything into consideration.