It's a neg, so the longer/slower scans help the highlights, which is where you see the most grain and washout.
I didn't really compare properly, but subjectively the microtek seems better than the 4870 at the darker areas of negs and slides. Especially Velvia/Kodachrome where things get really black, but there's a still lot of detail. Performance seems to be limited by noise, which is really down to insufficient light - just like a digital camera. Here I think traditional darkroom is better. You can increase the exposure without incurring the noise penalty. Drum scanning is another possibilty... But the Epson results look good, so probably not warranted.
I think the real answer is a design change on the scanners - dual pass, one with a higher lamp output setting to resolve the dark areas of film. And a pass with a lower lamp setting, then blend. Software wise we're there, but the hardware needs a variable level light source that doesn't change colour as it changes intensity.
Jo-1 scanned one of my negs on his minolta 5400. That was able to resolve all the dark areas of the neg without blowing the rest. Really impressive.
One other thing that may be affecting you is if you're using 16bits per channel or not. I'd guess you're doing that as a matter of course, only switching down to 8 bits for web use. If you check the specs, I think you'll find that internally the Nikon uses fewer bits/channel than the Epson (24/48 maybe), and this would also affect the darker areas of the film adversely. (There was a really good article on bit depth and how it affects dark areas of an image on Luminous landscape a few years ago, not sure if it's still there.). Net summary, the smaller the bit depth, the higher the level of posterisation, which is most noticeable in the darker areas of the image (here read film).
Edit - doesn't look as if the article is still there. Antoher thought is the colour space you're using. as soon as you move an image into a big colour space (like prophoto), your small colour space image will be compressed and lose detail. This will tend to increase the posterisation.
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