As stated above, Fir will generally be heavier, just be sure to get matched shafts as fir weights can vary quite a bit. Sitka Spruce is a bit lighter than the average cedar, but is pretty consistent in weight. Both make very good arrows, altho IMO fir is probably the best shooting wood arrow species. It seems to recover quicker than the others. It also tends to be more brittle than cedar or spruce, so check your shafts carefully and often. Both the Surewood Fir and Hildebrand Spruce are very nice.