i have used both, but i prefer sitka, its incredibly tough, and when i want a heavy hunting arrow- i can use heavier broadheads, and still get up to the fir arrow weights- yet have a far better FOC percentage.
my stumping arrows are spruce, and i usually lose them before i break them :D
another great thing about spruce is that it has a very pronounced "inter fiber connective tissue"component.
in other words, transverse micro fibers that connect the longitudinal fibers of the wood.
it is classified as having the highest strength to weight ratio,i guess thats why they use it both for spar wood in the boating industry, and as aircraft framing wood!