My experience with epoxy floors is years back in production shop settings. They didn't hold up well. I'm sure that they've probably improved over the last decade or two. I think they look really nice but the first scrape or tread stain would kill me. I'll be just fine with seasoned semi-slick concrete.
Steel on slab would be nice and there are plenty around here. The reason I went polebarn was mainly cost. If I could afford stick built, that would have been my first choice. As it is now, I think it'll be fine. I may come back later and put in a ceiling and blow-in insulation above it but it's all insulated now. I'll have to work a couple years down there to see if it's good enough as-is.