All depends, if he's a mech dismount type that gets about by way of APC, he might prefer the stouter BAS for use as an entry/egress tool in door to door type stuff. If he walks a lot of foot patrols, every ounce counts, so the Siru would get the nod.
The guys who pointed out the low probability of hand to hand combat with edged weapons are absolutely spot on. For a modern soldier's field knife, think tool first and weapon as a secondary consideration. Our boys are pretty well armed when it comes right down to it, but a sturdy dependable knife is always a welcome addition to one's field gear.
I never drew any blood with my BAS in Afghanistan, unless you count camel spiders and such, but I did put it to good use rescuing a BBC crew's gear from under a fallen tree after a violent windstorm. The BAS earned it's keep, and we successfully extricated their expensive equipment, along with what was left of their tent. I'm a fan of the 15" Siru for fast moving "quick trips", but there's times the added ounces of a BAS, or even a 15" AK, are more than welcome.
Sarge