There is no general answer for your question, especially if you donīt explain your situation more explicitly.
If you are eager to grow quickly and donīt fear deep red numbers in income statement, take every loan you can get and go shopping. If you are looking for the safe way, try to earn enough cash first. If you are very sure you will always find a task for your aircraft, buy it. If you canīt be very sure how your market will evolve or that you will face fierce competition in near future, go for leasing short term, this way you will get rid of your aircraft very easily.In the other case you must hope to find someone to lease or sell your aircraft to.
In every case make sure to keep up cashflow, if you canīt pay leases,loans or wages when they are due, your airline is doomed to fail quickly.