I can't give you the differentiations between the leaf and tip anatomy of Val and Sag, but there are some. The giveaway is the great diference in their flowers - Val is all but petal-less, Sag is actually an attractive small flower (some hardy emergent forms are even showier). Val has single "blooms" per spike, Sag spikes divide/branch and produce a continuing series of individual flowers over an extended period.
I use some deep subtrates without issues - in heavily planted tanks, depending in part on the plants used, it is not so much of an issue. The roots of Val and Sag and Crypts and some others tend to be multiple, long, and vertical. Deep substrates will allow for healthy strong stands of these plants.