According to page 186 of the manual, the Nikon D70 can tolerate any flash up to
250 volts running through the hotshoe, and according to:

       http://www.botzilla.com/photo/strobeVolts.html

the Sigma puts out 4-12 volts, and the Cobra puts out 66 volts.  Note there is
a caution about negative tip voltages in the manual, but I'm not sure how many
flashes are that way (I suspect it is pretty rare).

This means either flash could be used.  Note, using a slave is probably not a
good idea unless you are the only photographer there, since the slave will see
the other flashes and get discharged.

Here is a flash tutorial I just ran across:

       http://www.camerahobby.com/Ebook-FlashTechnical_Chapter9Sub.htm