I've had 2 incubators so far, and they happen to be these two specific incubators. As always, your mileage may vary, and these are just my experiences.
I started out with the 4250 and got 2 or 3 good hatches out of it. I noticed however, I suddenly started having terrible hatches with it. By that, I mean the hatch rate was low and the chicks that did hatch were taking longer than normal to hatch. I wasn't doing things any differently. The incubator apparently is now running a couple degrees too low, and the onboard instrumentation is not registering that. We also can't seem to keep the little black plugs on the sides (where you add water) to stay in. That'll definitely mess with your humidity levels, and could possibly be why that incubator is just not heating properly. Dealing with humidity with this incubator was a bit on the frustrating side. I loved the automatic turner. It did an excellent job, though it did heat up the corner noticeably in which it occupied fwiw.
Since that incubator was not working properly, I bought the Nurture Right. So far, loving it. I've only hatched one set of shipped eggs, and had a reasonable hatch rate out of it. It was so much easier to see the chicks hatching. Very much like having a front seat view. Its onboard instrumentation is accurate thus far, and I found regulating the humidity to be much easier with this incubator. It doesn't have the massive amount of space that the 4250 has so if you put a ton of eggs in there (especially large fowl), the chicks will be crowded together. Something also of note is that the 4250 has a thin plastic shell with styrofoam inside. The Nurture Right is made with plastic, but seems more sturdy to me.
That's all that's coming to mind; I hope it's helpful.