I started with the D300 and since moved to FF with a D700 and D4. I have kept the D300 sitting in the closet. I have been looking at some of the mirrorless cameras. How does the D300 compare with Fujifilm's lastest mirrorless?
Your question has been up for a while, with no answers. I don't have a direct answer either, but I have a D300 and an Olympus OM-D E-M5, and my impression is that while both look good at ISO 200, when you push them to ISO 3200 the E-M5 has a cleaner image. I haven't done side-by-side tests on the same subject, but I plan to, to see if that impression is true... and I'm pretty sure it is.
My guess, not having used Fuji's mirrorless cameras, is that they are probably at least as good as the E-M5 in this regard, so my thought is that you'd get similar quality at low ISOs, but at higher ISOs the Fuji images would look better.
I'm thinking of just using my D300 when I don't need to use it for low light images. I find the weight of the D300 with my 28-200mm lens very manageable and so getting a new mirrorless will not provide me with much benefit.
I sold my D300 to fund the purchase of a Fuji X-E1. From a high ISO standpoint, the Fuji is somewhat better. I don't know the exact point the Fuji pulls ahead, but it's probably around and above 1600.
Otherwise the cameras are very different. Obviously the Fuji cameras are smaller and lighter, with different handling. The Fuji start up time is noticeably longer. There is shutter lag. EVFs take some getting used to. Focusing isn't as fast. There is focus peaking for manual focus lenses. I really like the live histogram in the viewfinder.
A Fuji in hand is better than a D300 in the closet, particularly if small, light, and casual is appealing at times. Don't get me wrong, I still have D700 and several AF-S lenses. I don't think mirrorless replaces DSLRs, at least not yet. There is room for both.