In my team, some members use Fireworks and others use Axure.
the tools used to create prototypes are of lesser importance, as long as the end results communicate the ideas efficiently towards the stakeholders. Leave the choice up to the designers to decide which tool they want to use. They will produce results faster, more time for evaluation and rework -> results in a better experience.
Keep in mind:
if a lot of handovers occur inside the team, might be best to go for one tool. This way everybody knows this tool and work is easier interchangeable.
Working in smaller organisations it is often cheaper to head into one direction (licencing costs).