I didn't notice much difference between XR and Marathon Plus. However, I wasn't particularly sensitive to any differences here.

The trip is documented at http://www.bikerussia.com

As far as roads go, sealed roads in the east were at least as good as sealed roads in European Russia, where occasionally the underlying concrete/asphalt road base was poor and there were many trucks. However, as you mentioned there was ~1700km of unsealed roads between Chita and Khabarovsk. I found some of those unsealed roads to be more challenging than unsealed roads I've cycled in Alaska and Northern Canada. There was a tendency for the road to be both too hard (washboard) and too soft (lots of loose gravel graded on top) at the same time. There was also a surprising amount of traffic headed westbound as used Japanese cars had been imported and then were being driven to Siberia. This meant lot of dust. I cycled 700x35 tyres which did work a little better than the 700x28 that my riding partner used.