I own the Rotel 1068 but have not compared it directly to the 1066. The 1068 has some features, processing modes and updates in digital circuitry not found on the 1066 and which cannot be added by updates. The review of the 1068 in July 2004 SGHT
http://www.guidetohometheater.com/surroundsoundpreampprocessors/704rotel/index.html catalogs differences from the 1066 in features, components and performance. Whether these changes are important in your system or justify the price difference is your call.
Neither unit has HDMI connections, which it is rumored will be included in the next generation Rotel units likely available at the end of 2006. As the introduction of the new Rotel processors approaches, prices for both the 1066 and 1068 should decline.
Another site hosts a recent discussion of 1066 vs. 1068 that may be of interest.