6, 7 and 8 are the same width; 9 is thinner and 10 thinner yet.
You can use thinner (9/10) chains on 5/6/7/8 drivetrain, though I suppose there is maybe more risk lf the chain jamming between chainrings or something. But the key dimension, that is, the inner dimension where the teeth go, are the same for all the chains. 9/10 speed chains needs to be narrower on the outside dimension because of the closer cog spacing (fatter chains would rub adjacent cogs).
6/7/8 speed chains are cheaper; I concur that the SRAM PC 58 is a great value. I really dislike the pin-replacement requirements on Shimano chains, and won't buy another Shimano chain if I can help it.