Eric,
Nighttrain has a good point. My dad's barn was always 1/2 dirt and still is. It's fairly workable if you've got big equipment, more of a problem if you like to keep things neat. Machine storage and stalls are dirt, work areas are concrete.
It's probably critical to have a good concrete work area. A lot of the older "gravel" floors I've seen end up mostly being tracked in sand. Getting sand all over a greasy wrench is never fun, and loosing a nut or washer in the sand is sure to slow you down.
If I had to go 1/2 and 1/2, I'd probably put some thought into making sure I was set up for an easy pour to finish the job. Additionally, I'm gonna just throw this out there. You could also consider interlocking pavers to finish it off in the future as well. We don't use them a whole lot in the US, but if you put them over a strong gravel base they've got some pretty good properties. We seen them on airports and shipping ports, so a barn shouldn't be too much of a stretch either. If you don't mind doing the work, you can actually do it fairly cheaply too. I'm not suggesting they're the solution in every case, but they do have applications. Worth considering, even if only long enough to totally dismiss.
Joe