If you are in earthquake country, I'd go with the piers and subfloor idea.

For a workshop concrete can result in a lot of discomfort and sore knees, especially as you age. There are rubber matts that help but they only help a little. For that reason I'd go w/ the joists. However, you need to consider your climate, critters getting under it, etc. What are your plans for heat/cool? Will you need to run ducts for HVAC or dust collection?

For my new shop (in Minnesota) I did floor trusses w/ 3/4" ply sub floor, gypcrete w/ radiant heat tube, and 3/4" solid oak (remnants, scraps, etc.) flooring.

Here's pouring the gyp (the stringers are for attaching the wood flooring):

Cool! I'm in Southern California so don't need heating. I plan to put a heat pump water heater there, which will be producing cool, dry air as a byproduct. I hope that should be enough for cooling.

I'm thinking concrete slab would protect the house from souterrain termites and mice.

This is how it look right now.