I use both. My wife has a D50 and I have a D70s. The D50 has lower
noise. Here is a test I did a while ago.

http://web.aanet.com.au/pics/docs/d50_d70s_noise.htm

I like the two control wheels on the D70s and the DOF preview. The
other significant advantage to me is that the D70s internal flash works
as a wireless commander with my SB600.

According to Aeroadster  <Aeroadster.25yro0@news.photobanter.com>:

    Hmm ... unless the D70s has changed recently, the kit lens for
the D70 (and D70s) is a single lens -- 18-70mm f3.5-4.5.  The two-lens
"kit" only was made for the D50.  Though vendors may elect to offer with
different lenses than should come with the camera -- both Nikon and
non-Nikon lenses.  Are you *sure* that your vendor is offering the Nikon
lenses?  I believe that the 18-70mm kit lens for the D70 is more highly
rated than the two (much less expensive) lenses for the D50.

    I *believe* that what is really happening is that the default
settings in the D50 produce a crisper conversion from the internal raw
format to .jpg -- aimed more at the entry-level photographer who will
never progress to shooting in raw and custom processing them.  It really
isn't sharper when you go to examining the details down at the pixel
levels, but it *looks* sharper in the overall image.

    The default .jpg conversion in the D70 and D70s is aimed more at
preserving more of the image range to allow the photographer to have
more choice as to what will be shown -- after the picture is taken.

    Probably.

    And, you get more flexibility with the D70s, because of the
greater number of controls -- in particular the two wheels (one for the
thumb and the other for the middle finger beyond the release button)
which allow better (and more convenient) control of shutter speed and
aperture when in manual mode.  With the D70 and D70s, the thumb wheel
controls the aperture and the middle finger wheel controls the
shutter speed.  I don't remember which the D50 gives you on its single
wheel.

    Also, when you're pressing the "Qual" button, the thumbwheel
lets you control the jpeg compression (fine, normal, basic) and to also
select raw mode or raw+basic, while the middle finger wheel allows you
to select Small, Medium, or Large image size.  You *can* select these
from the menus, but if you want to change in a hurry, having the two
wheels makes things a lot nicer.  There is so much in the menus that
finding one particular option may take longer than you expect --
especially if you don't do it often.  And sometimes, the limited space
for descriptive text in the menus may still send you to the manual to
figure out what a particular option does -- such as the "Shutter Speed"
(option 21), which does *not* actually set the shutter speed.  Instead,
it sets the slowest shutter speed at which flash will be used in P and A
modes.

    You *can* get explanations of some of these by hitting the '?'
button while a menu selection is highlighted.

    And I believe that the D50 has no button to allow you to select
backlight on the LCD control panel on the top of the camera, nice for
setting options in the dark.

    I've only handled the D50 once, and aside from what I mentioned
above (which is partly from the reviews read in the light of my own
usage patterns), I also seem to remember the viewfinder area being
smaller with the D50.  (It also seemed darker -- but that may have been
because it had no fully-charged batteries in it in the store demo model,
so the lens had closed to minimum aperture.

    Well ... you have my view of some of the differences as viewed
through my own perspective.  Needless to say, what I actually *have* is
the D70 (not the D70s), with the firmware upgrade installed at the first
opportunity.

    I hope that this is at least some help.

    Good Luck,
        DoN.

Blimey. I have a D50 and I love it but if I'd been offered a D70s for
the same price, I would have grabbed it so fast he'd have to be careful
not to lose his fingers.

HTH
Tom