This isn't an easy to answer question, because it depends on many factors.
On the one hand, you are right, you don't have to pay leasing and you can use it for loan.
On the other hand you have this aircraft. And maybe there will be one point you don't want it anymore - so you try to lease it out or sell. And here's the problem: Sometimes nobody wants your aircraft.
In your case it's DC-6(B) and DC-7, I think this is ok in this scenario.
Also some notes in this;
Modern aircraft are very expensive. In early games (Early Days, partially Jet Age) it is possible to direct purchase lots of aircraft in the first 18 month ingame a game world has started. This is not (this easy) possible in later game scenarios.
So, nearly all games at the moment last 20 ingame years. Buying or leasing will be much more interesting when games start in the Early Days and run for 60, 70 years or longer.
Long text, short result;
If you purchase the right aircraft at the right times it's good. But it also could be very bad for you.
If you lease aircraft, you can get rid of them fast if you don't need them anymore, even it's a very unpopular model (due to increased fuel price for example). On the other hand you pay much more money if you operate it more than 80 month (new aircraft).