Is there a major cost difference between adding a fire escape vs. adding a deck?

As new landlords, we learned that we need to add fire escapes to the back of our 2 unit, 3 story building. We’re debating just putting in decks instead of fire escapes, since these add more value to the units.

What is the approximate cost difference between adding a fire escape vs. adding a deck?

Without seeing the layout of your units I would say adding fire escapes would be cheaper for this reason. A fire escape can be built from the ground up and supported by carefully placed supports placed in concrete at the base. Existing units would require more support to allow for the placement of decks at different levels and may need to be tied into existing structure. Hope this is clear.