If I'm paying that much for Krug, I'd want prime Torey Krug. I'm not expecting his prime at all.
Honestly, I'm not all that enthused about Krug in general.
Via evolving-hockey (and hoping the link works the way I want), Krug and Petry had similar shots, fenwick, and corsi for and against, but Petry's xG and actual goals for and against were both quite a bit better. Part of that could very well be because the Penguins had a better defense as a whole (14th most goals against vs 6th most against), but certainly not all of it.
Additionally, Krug has two more years of term, and has a full NTC for the next two years as opposed to a 15 team NTC.
So we add a 1st, 3rd, and young puck-moving defenseman to overpay a defenseman for 4 years instead of 2 years. I'll take my chances with Petry, thanks.