I'm not in the Modern Times, but I'm sure this is valid for all game worlds;
Is it an aircraft you will for sure fly till the game world ends, purchase it.
Is it only for some years, maybe if a better aircraft is released, then only lease it.
While you can calculate for yourself how long you must operate an aircraft then leasing costs are higher than the purchase costs (there is nearly no interest in AWS!), think about the fact too, that you can't scrap aircraft younger than 20 years.