Cap Impact of #6 or vet (e.g. Briggs)?

[freddyg12]

I posed this question at the end of one of the marathon Briggs' threads;
For our resident capologists, which will likely impact the cap more, signing the #6 pick or Briggs or some other vet?

Let's assume that there will be significant inflation in the top 10 rookie contracts this year. If the #6 gets the same guaranteed money as Briggs would, how does this impact the cap? The reason I ask is because of the requirement I believe that teams have to allocate a certain percentage to rookies. I'm not sure how that works or if there is much of any difference, but in year's past I recall us never being concerned with having sufficient cap space to sign high paid rookies. Of course trading down would save us $ & gain more roster spots.

Any input is appreciated.