With the scanner, 600 dpi. (For a while I cranked it up to 1200 but did not notice any improvement.)
With the copy stand, I am using a Samsung NX30 to take 20M-pixel photos, using bracketing mode to take 3 shots of each original (slightly different exposures) and I just pick the one that looks best. (Using shutter priority of 0.5 second, which puts the aperture at f/6-12. This seems to work the best for capturing detail.)
The quality (scanner vs. camera) comes out the same, despite the theoretically doubled resolution with the camera.
Camera advantages:
-- much, much faster
-- easy adjustment on the rare occasion I need to boost exposure
-- press one button per print (scanner needs 2)
Scanner advantages:
-- it gets brightness right more often
-- I don't have to take breaks to recharge
So far, so good. So why am I thinking of sending stuff out? When I take a very close look at the photo vs. the file, I can always find details that did not get captured by the camera or the scanner, even at their highest settings. I have at least a couple hundred photos for which I'd like to have the best copy possible, which should be achievable from the negatives scanned at 3000-4000 dpi, which is the resolution that most of the services advertise.