I'm by no means an expert so take what I say with a grain of salt. I would suggest that any combination of x/K wants to stat some pdef for the added advantage of enhanced armor..Unfortunately, this does nothing for defense against magical attacks.
My more recent experience in seige has me thinking that M/D and M/Wd are pretty wicked, arguably as good as P/S. Their huge advantage is that no physical DPS has any good method of stacking Mdef.
I play a R/S or S/K in seige. As a R/S, one on one, I have no issues with Mages or Priests (not all the time but a lot of the time) mostly because they cant see me. If there is more than one, I might as well just lay down and die. When I come out of hide and attack one, the other kills me (yes I travel alone a lot). Grouping is your friend here.
As a S/K, I dont know, I'm still working on my gear but it comes down to who get the jump first. A M/D, P/S or M/Wd who is paying attention can kill me very quickly.
Statting for R/K and S/K would be similar. Stam, dex, patt and some def for seige.
Just my .02