In the summer of 2008 I went with my family to Mongolia. Came in by the transiberian to UB. The Three Camels in the Gobi is well worth it. It was the best place that we stayed at during our three and a half weeks in Mongolia. The Teralji Hotel outside of UB was definitely not. Waste of time in Mongolia.
Mongolia will mean very little unless backed up by reading before you go. In UB all hotels were pretty dumpy. Both the Museum of Natural History and The National Museum of Mongolian History are solid. See them quickly and bolt out of UB. You would prob fly instead of drive anywhere. You don't have enough time to bother with the long drives between places. The airline www.eznis.mn was well run and on time from my experiences. If you are a fly fisherman then 6 days in Mongolia is ideal. My husband had an incredible time with an exceptional guide. If you need the info I can get it. This was just a quick response. We actually covered a great deal so if you need it flushed out some more I wil be happy to fill you in.