Concrete is one of several trades that have been doing substandard work the majority of the time because the customer demands it cheap, that no reinforcement and no sub base has become standard. The concrete guys I had pour my basement slab, hadn't seen a crushed stone/compacted sub base in over a decade, everyone throws a few inches of pea or pours directly on dirt.
If the sub base is correct (8-10), compacted correctly, graded properly, the concrete placed correctly and not to wet, and the slab cured correctly, you may be fine without reinforcement. If one or more above condition are not met, rebar may be the difference of having a small tight crack and having a 1" lip when the edge of the crack shifted.
Will you maybe lay on the floor while working on equipment? You may not like fiber as it will cause many to itch.
I would suggest 3500 or even 4000 psi concrete, the extra cost is small for the extra strength.
There are lots of option, even steel fiber which is a substitute for steel reinforcing but you concrete guys might not want to use it. I wouldn't even be scared of a 5" slab if sub base was correct and with the reinforcing.
Really you just have to make a decision and go with it.
