We just returned from our Baltic cruise (Constellation May 22/June 5) and visited both Peterhof and Catherine's Palace as part of our Red October tours. If push came to shove and I had to choose one or the other, I'd opt for Catherine's Palace. While the grounds of Peterhof are beautiful, a visit to the public rooms at Catherine's Palace really gives you a feel for the lifestyle of enormous wealth and grandeur of the Russian czars and aristocracy. The Amber Room alone is worth the trip.
Also, Catherine's Palace was bombed by the Germans during WWII and left in ruins. The restoration that has taken place is nothing short of spectacular, especially when the beauty of many of the rooms is contrasted by actual photos of those rooms prior to restoration.
We loved both. Is there no way to do both? Are you going to the Hermitage too? You will not be sad if you make either choice. That's like choosing between a banana split and a hot fudge sundae. Hmmmmmmm?????