Fir and spruce are different plants, and have slightly different flavors, but both are citrusy, piney and, in high doses, bitter. Either works. You could experiment with chopped pine needles or juniper branches, too. This beer is pretty "hoppy," and I use quotation marks because some of that will come from the spruce or fir tips. Hard to determine exactly what the IBUs will be, but it'll be more than the 52 listed here. You can of course reduce the hops addition if you want a less pungent brew.
I like the mix of Scottish ale yeast with this beer, but any subdued ale yeast is fine. Avoid Belgian yeasts.