I realize this thread has gone a bit stale now, but I felt I should comment, since I have direct experience with the question the OP had.
I have thousands of slides and negatives that I've wanted to digitize for a while now. I began to digitize them with the first scanner I owned that had enough resolution to consider doing so. It was an Epson 3170, with a max optical res of 3200 ppi, and it did a decent job, but I could tell when examining my slides under a loupe that I wasn't getting all the detail. So eventually I upgraded to an Epson 4990, which has a claimed max optical res of 4800 ppi, but which in fact doesn't have a much better resolution than the 3170 does. I determined to my satisfaction that my 4990 scans at about 2000 ppi. On a good day. Which it turns out is about typical for a flatbed scanner. Even the vaunted Epson V7xx series don't put out much more resolution than the 4990 does. And of course, when examining my slides with a loupe after scanning with the 4990 I could see that I was still not getting all the detail that was there.
And then I found out about "digital slide duplicators". At the time, they were all over eBay. Probably still are. I found some info on them and the results looked really good, so I bought one. Problem with using a "digital slide dupicator" with your DSLR is the resolution is dependent upon the zoom lens you're using. And chances are you'll need to use a zoom unless you own a full frame DSLR because the focal length you'll need --- or the one I needed anyway -- is about 70mm, maybe just a shade less. So, I was using an EF 28-80 zoom, which was just okay, and to make things annoying, every time I inserted a new slide in the holder, the camera would refocus, so I'd have to manually straighten the slide because the front element rotates, donchaknow. Well, I persevered, though, and shot a bunch of dupes using my 28-80 lens and they were definitely, without question, sharper than the scans I was getting from my Epson 4990. But using that 28-80 zoom was bothering me. I had a growing suspicion that it was the Achilles heel in my setup.
So I got industrious. I removed the slide holder from my duplicator because it wasn't adjustable. Nowadays the ones I've seen are, which is good. And I modified the duplicator tube so it would accept a stage. I had previously bought an 80's vintage zoom slide duplicator with both slide and roll film stages, and I used these. Next, I removed the inner corrective lens from the duplicator. The tube has 52mm threads, so I just threaded it directly onto my Micro-Nikkor 55mm f/3.5 lens. To get things to focus where the slide or negative was the right size, I had to use extension tubes with the Nikkor 55mm. In my case, I needed 30mm. So that was the totality of my outfit: an adapter for my EOS camera to the Nikon F T-mount, 30mm of tubes, the Micro-Nikkor 55mm lens, the optical tube from the "digital slide duplicator" and the slide (or negative) stage.
This setup has worked great. For one thing, the lens's front element doesn't turn. For another, it's a flat-field macro and it's extremely sharp. I use my camera's Live View function to nail focus, stop the lens down to f/8, set up a off-camera flash, and fire away. And yes, the images from this rig are noticeably sharper than they were when I was using the 28-80 zoom.
Now, my DSLR is just a cheezy, entry-level Canon with 10.2 mp of resolution. This translates into about 2600 ppi of true image resolution, which is better than any flatbed that I know of. But just think what this rig would be like with a T3i or a 7D, eh? And if you have a full frame camera, then you don't need to mess with all this cobbling together of stuff. Just get a good macro bellows with slide copier stage and a macro lens, and use that instead. Or a 1:1 macro lens would probably work, too, but you'd have to figure out a way to mount the slide or negative.
In an earlier thread, where I mentioned that I used this rig, pellicle commented that this setup couldn't compare to a scanner that you can load up with a bunch of slides or negs, set things up, click on scan and then walk away from the scanner for however long. Well . . . that's true. But the scans from a flatbed are inferior, for one thing, and for another, I can usually scan about three or four slides per minute using my rig. It takes longer with negs because repositioning them accurately takes longer. So, even though I can't walk away from my rig, I can make the dupes a lot faster, and they're better to boot.
So now that I've determined to my satisfaction that duping 35mm slides and negatives in some cases (b&w are easy, color can be a problem) works better than scanning them, I'm faced with this dilemma: do I go to all the trouble of archiving all my slides and negs with my cheezy 10.2 mp camera, or do I wait until I get one with better resolution? This question has become so troubling to me that I have decided to halt the process of archiving my images and wait until I buy a DSLR with higher resolution. With some of the images I have, my camera with this rig will resolve the individual particles of grain, but with others, especially Velvia, it doesn't resolve the grain at all. Hence my concern more than ever that I should get a DSLR with more resolution. Especially since, for what the high-end Nikon scanners go for these days, I can buy a much better camera than what I have now.
Sorry for the length of this post, but I figured I'd give you the complete rundown from beginning to end.