In the testing I've done the Young's modulus (E) along the grain for all softwoods tends to 'track' the density in the same was for every species. 60% of all samples I've tested fall within 10% +/- of the Young's modulus value predicted by the density. That's pretty good for a natural material. Since long-grain Young's modulus is the most important thing determining the thickness of the top, you can pretty well use any softwood for a guitar top, so long as you make it the right thickness.
Low density tops end up being lighter, so long as they fall 'near the line' on the graph of long-E vs density. Stiffness goes as the cube of thickness times E. Leaving the low density top a little thicker raises the stiffness faster than the weight.
In my shop my students and I have used a bunch of different softwood species. All of the spruces have acted like spruce: Red, White, Black, Engelmann, European, Lutz, Sitka, and maybe some others. Don't get hung up on the on-line stuff: spruce is spruce. We've also used Western Red cedar, Douglas fir, Western hemlock, White pine, Pondarosa pine, Redwood, Lawson's cypress, and a couple of others who's names escape me. Despite the published 'average' numbers, all the redwood I've seen that was worth using has been about as dense as the 'average' Red or Sitka spruce. The only low-density stock I've come across was some 'salvage' wood that had very low long-E and a tap tone like cardboard, which I sent back. I'll note that redwood can have what seems to be a lot of built-in stress, so you have to be careful, particularly if it's figured. Redwood, Doug fir, and WRC tend to lack toughness: they're 'splitty'. I don't use those for bracing. White pine has a very soft surface; use a larger bridge footprint and a good pick guard.
Note that when I say 'softwood' I'm using the botanical term. Some soft woods, such as Balsa, are botanically hardwoods, and don't necessarily follow 'the rule'.
Keep in mind, too, that there is a lot of variation is any wood species. You can't always tell just by looking at it, and labels don't mean as much as you'd like. There's no substitute for measuring the properties of the piece you want to use. Some hardwoods, such as balsa, and Spanish cedar, can be low enough in density and high enough in long-E, to work as top wood, and some softwoods won't. These days it's not hard to do the measurements, and both Gore and Hurd ('Left Brain Lutherie') give good instructions on how. Daniel Haines published articles on this in the 'Journal' of the Catgut Acoustical Society, which I believe are available on line, as did Morton Hutchins. There is other material on this general subject in that source as well.
We here in the US are blessed with abundant sources of all sorts of usable lutherie woods. The only one that's really hard to find a good substitute for is ebony: there are lots of hard close grained woods out there, but hard, close grained, and black... 
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