How would one decide on which to use either 3 phase or single phase power to heat an airplane hanger?

It would depend on what type of heater you are using. Generally speaking 3PH is reserved for commercial applications while single phase is for home use. Is this hanger in a commercial air park or at home? What sort of power do they have available?

Motors operate more efficiently on 3 phase power. Rule of thumb 1 HP or below single phase. 5 HP and above 3 phase. The middle is a grey area, served well with 3 phase.

3 phase can have consequences, both positive and negative with respect to cost.

It's not uncommon to have 460 V 3 phase heat pumps and 277 V lighting.

One would need to do a manual J and manual D calculation to determine the size of the ducts and BTU/hr.

But then there all all sorts of ways and methods to an airplane hanger. One could be just heating, just cooling, or very comfy working environment for repairs.

Just like anything else, you have to size the system and look at options.