My two cents - for obvious reasons I have used both extensively.
There are some pretty distinct differences...
The kestrel forward edge geometry is geared more for fighting than that of the Shrike (the edge is layed back more and the beard is a touch longer). Not to say you couldn't breach with it, just a better cutter for that sort of thing. The spike is also a little thinner than that of the Shrike.
Both easily handle body armor.
S-13 is 20 oz
Kestrel is 25 oz
If you are breaching you need to know that the exposed steel tang on the kestrel is conductive, even through conventional finishes. The S-13 is totally non conductive on the handle. Not too terribly important on the civilian side, crucial on the LE/Military side.
Both use the same carry system (ie. MOLLE, Weapon Sling, Belt compatible)
S-13 has a sharpening stone that goes up in the handle - there is a 4130 Chrome Moly tube TIG welded on the end of the handle so that you have steel going all the way to the end of the handle save 1/4" for conductivity purposes. This is where the sharpening stone is housed.
What is the final verdict for me personally? If I was buying one to carry downrange I would pick the S-13. Great close quarters piece that will take a ton of abuse for the stuff you really end up using it for most of the time. If I was buying one to keep in the car or around the house? The Kestrel, just because I think it is sexy.
Sorry for the long winded answer.
RMJ