I can't comment on the Blue Train but we travelled the Rovos Rail from PTA to CPT about 2 years ago. The main differences, as I understood it at the time, are that Rovos uses antique refurbished carriages (beautiful) and steam engines (at least to pull you out of the station) whereas the Blue Train uses luxury rolling stock from the 1970s (updated). Also Rovos takes two days to make the journey to/from Cape Town and includes two off-train stops at Kimberley and Matjesfontein - the Blue Train does not stop for sightseeing. These were the reasons we went for Rovos in the end but it will depend on your preferences.

The accommodation is beautiful and relatively spacious even though we went for just a standard cabin. There is a shower and minibar (as well as room service) and a full double bed. The dining cars were just stunning, dating from the 1920s, with intricate wooden carvings and columns. At the rear of the train there is a bar and observation car with an open rear deck - we had a magical evening after dinner one night sat out there alone as the train travelled through the moonlit Karoo! And the quality of the food and wine is first class.

For us there was only one downside to travelling by Rovos and I don't know whether it will be an issue for you. Basically we are both in our early 30s and the average age of the other passengers was 60+. While everyone was perfectly friendly we tended to keep to ourselves as we had relatively little in common with people who were retired etc etc. It certainly didn't affect our trip but we did feel very conspicuous (being young and a gay couple!)

Hope that helps.