Assuming you are comparing the Dimage 5400 to the Nikon 4000, I don't know either [img]/ubbthreads/images/graemlins/grin.gif[/img]. I've posted some queries here and elsewhere asking the same question, and still don't have an answer. All I know is the 4000 does some great scans, but can be tricky to operate. I never used mine - a friend from overseas was visiting me and bought it from me, and took it with him before I ever had a chance to take it out of the box, so I'm still using my old Coolscan III. One advantage of the Nikon 4000 is that you can get the auto-feeder for it, which will hold and automatically scan 50 slides at a sitting (or overnite). I have a friend who did this to archive his old slides (which is why I was looking into it) and said it did a great job. The 5400 will only batch 4 at a time, but it is about $400 cheaper. Neither comes with the Silverfast 3rd party scanning software, which I am told makes a big difference in quality and ease of use.
Unfortunately no one I know owns both (I don't even know anyone with the 5400) to allow for a good solid comparison. You have to rely on professional reviewers, so you might try looking here:
I'll be curious to know if these help, and which one you end up with.
Andy Pearlman Studio