The answer to your question lies in that annoyingly familliar and oft-repeated line that I use quite a lot "It Depends."
If you feel you can justify the one year drop. And I don't mean in the PI. You first have to crack the entrance to get a call and then you get to the PI. So justify the drop first by working your ass off to crack that entrance, (Hey I can tell you that's NOT what I did

, otherwise
"I could have been somebody." and then Brando turned in his grave

)
On the other hand, there are people who as Digital_Saint has mentioned juggled both things quite well and came out on top with good work ex and that elusive dream most of us know as an IIM call.
Its you who have to decide based on your personal strengths and weaknesses & your job (profile, satisfaction, experience, learning - cumulative score of all these

)
Don't know how much this will help but even so...