What are the advantages to using fiber or wire and which is the strongest for a 40' x 60' garage?

Wire mesh has the advantage of being commonplace, but does nothing for the concrete strength; fiber has the advantage of being simple to place, increasing concrete strength, but is difficult to float/finish.
In my opinion, this is an instance where less is more - use fiber.
Mesh needs to be elevated and correctly placed vertically - about midway in the thickness of the slab.
Another consideration might be that removing a fiber-reinforced slab is a job for Superman; while Mighty Mouse will be able to break the wire mesh floor!