Hi Jacob,
I'm using Douglas fir for probably the same reason you're considering: the borgs around here don't carry SYP 2x construction material. All the 2x material here in NJ is douglas fir. Actually, there is pressure treated SYP available around here, but I didn't want to deal with that stuff.
I think that SYP has been elevated as a workbench material for the wrong reasons. Chris Schwarz seems to like it for the following reasons:
1. It's cheap, and available where he lives.
2. It's stiff.
3. It's pretty dense.
4. It's cheap, and available where he lives.
He has an interesting take on the use of beech in traditional European benches. Beech is stiff (not as stiff as SYP or Douglas fir), dense, and cheap -- in Europe. It's expensive here, and hard to get, so for those reasons it's not a good workbench wood on this continent. Maple is stiff and dense, but it's expensive, so SYP wins out here. (Actually, maple is slightly less stiff and less dense as SYP, much to my surprise.)
As far as stiffness goes, Douglas fir is a hair stiffer than SYP. It's not quite as dense, but there are other ways to make sure your bench doesn't move besides mass. But most importantly, around here it's cheap, and available.