There are a ton of considerations, starting with 'Doctrine', then tactics, other equipment, integration, other kinds of dependencies. So many things.

Often there are acute, specific needs, aka Canada has to land these all the time in the arctic, you need bigger hangars for the bigger gear, maybe we need 'more units for wider coverage' over large land mass.

Some gear is better v Russians, some gear for China threat.

Etc. Etc.