Well, for me it would totally depend on the wool being used in both blends... how soft is it, how well will it wear, etc.
Also, do you want it to be absorbent at all? If that's a big factor, you'd want to stick with the pure wool, probably, since it will absorb a little better. When DS was in diapers, we used wool longies (knitted wool pants) and they absorb quite a bit.
Also, is there a difference in caring for the two? You MIGHT be able to find some washable wool long johns, I think that might be less likely with a silk/wool blend.
All things being equal, if I didn't care much about the above factors and was just shopping online, I would probably get the wool/silk blend because they would probably have a bit more luxurious feel. That's my $.02. Hope this helps!