As the original poster, thought I'd give you kind people an update. In the end, we booked Silversea Silver Whisper because it departed from St Petersburg. Almost everyone we know (and some of you posters) stressed that you never have enough time in St Petersburg so it seemed starting there would solve the problem.

We flew BA First Class (God bless points and the 2-4-1 certificate!) into Moscow and spent three nights at the Park Hyatt. Wow. Took the high-speed train to St Petersburg and had two days of touring. The third morning we toured and in the afternoon we boarded the ship. The fourth day we toured again and the ship sailed at 7 PM.

The only downside is that this itinerary did not include Talinn and we would have loved to see it but having the extra time in St Petersburg - and making it easy therefore to include Moscow) clinched it.

As for the ship, another wonderful Silversea experience. Comfortable suites, a lovely bath with two sinks and a stall shower (in addition to the bath tub - does anyone really take baths?!), wonderful food, wine and liquor and superbly gracious service. Best way to sum it up is that we loved it so much that we took advantage of the discount and booked our next cruise for July 2013.

Thanks again for all who responded.