But then you wouldn't have a fairly fast approximately 50 mm lens. You'd
have a fairly fast 75 mm lens.
I would really like a fast 70 mm lens that would work as a fast 105 mm for
portraits. My Nikkor zoom that came with my D70 will, of course,
approximately equal a 105 in focal length, but not in speed.

A fast lens is always a nice thing to have, but perhaps not nearly as
important as it was in the days of film. Don't forget that your D70 can
shoot at up to 800 ISO with very acceptable results. The difference from
ISO 200 to ISO 800 is the equivalent of two f stops. That's the equivalent
of making an f4 lens as fast as an f2 lens, all other things being equal.
Still, there's nothing quite like fast glass, and f1.4 is pretty fast.

As for the focal length, unless you are doing tight work, in which case you
should just get a dedicated wide angle prime, I doubt that you'd really see
that much difference in usability between the 30mm sigma and the 50mm Nikon.
Assuming you have the standard kit zoom lens for the D70, compose a shot at
30mm using the focal length markings on the lens, and then frame the same
subject with the lens zoomed to 50mm. Not all that much different in my
book. When using any prime lens, remember that the zoom function is always
implemented with your feet.