IMO your decision should be based on whether you want an open car or not.
From what I understand the Cayman is a refined and tightened-up version of the Boxster with a bit more sportive handling.
But mind you, the Boxster is one of the finest sportscars out there already.
I would say: If you wanna track the car and really want to explore the extra ummpf and handling the Cayman offers, go with that one.
If everyday sporty driving and the occasional track-day + you like to enjoy the fair days in an open beauty the Boxster is for you.
Hard call. I'm faced with the same decision myself - as I was thinking about getting a Cayman in a year or two. However, we have 27 degrees here in Denmark right now, and I would hate NOT to have the Boxey on a day like this

Good luck with the eenie-meenie...