Couple of things, do I think Mercilus plus extra picks and a lower cost is a better deal than Spencer, no. Spencer is a known comodity and Mercilus is an unkown who only produced 1 year in college. He has just as good a chance at being Aaron Maybin, Robert Ayers, Vernon Gholston or Derrick Harvey as being the Lamar Woodley. And anything short of that would be a waste considering we have a solid player in Spencer. We have guys starting at G, C, DE and S who should be back-ups at best and some of the players at those positions are a better value than Mercilus, so unless he's a pro-bowler we at best traded 6 fpr half a dozen.
As far as next year, I do think we'll end up keeping Spencer. I think he'll have another season around 6 sacks, maybe 8 or 9 if we can get a better option than Kenyon Coleman at DE and he'll be signed for a number both sides are comfortable with.
If his demands are outrageous, I have no problem replacing him with a first round pick, however there's no logical reason to believe anyone picked in the range you described would be noticably better anytime soon.