Well buying planes creates a new revenue stream, while buying stock improves company value, but does not create revenue per se. When a company buys back its own stock, the shares are retired and cease to exist.
Both are valid tactics depending on where the company places it's future interest. If they want to raise the value of the company (their market cap) sometimes buying back shares, say $50m worth, can increase the value of the company by more than 50m. The company value may go up by $200m. That's a fair return but it does not further generate revenue.
Alternatively, buying planes increases revenue, and doing so pays attention to the YOY trend. This can also increase the value of the company by some amount (usually some capitalization rate which is not too high for AYR right now).
It's hard to say what they're going to do. I think either way works great, but both have a fundamental goal: increase the value of the company.