Having the choice myself, I lean towards the BATAC. It is just a little more user friendly size. In all honesty, I carried a ST-Ripper, a SJTAC Variant and a Dumpster mutt on my last deployment (not all at once). Of those, the Dumpster Mutt was the best size for what I was doing. Now to be completely honest... I used the folding knife in my pocket 95% of the time.
I carry a fixed blade on my kit for a tool, and for my last line of defense. If I am going down in a blaze of glory I can picture no bettter exit scene than me slashing away with a ST-Ripper.
If you are buying a fixed blade for a soldier, buy him one that is a comfortable size for him (or her). I vote BATAC, but if your friend is XXLarge, the SJTAC may be a better fit. Also, personnal taste plays a huge role.
Good luck finding the right knife and thanks for putting a Busse in Harm's Way!