Most here would agree that this is a question almost impossible to answer without a crystal ball. But I operate off of common sense and my common sense is still hanging with R22 for the simple fact it will still be around while this sytem is wearing down to the point it will need replacing again in the coming 7 to 12 years. At that time all this refrigerant stuff should have settled down and what works best will finally be known.
Plus you can always buy and extra bottle of 22 at todays prices and set it aside for him if you are worried about price or availability.
"The American Republic will endure until the day Congress discovers it can bribe the public with the public's own money.
- Alexis de Toqueville, 1835