Trans-Mongolian rail/flight to Ulaanbaatar

I am planning a trip for summer 2014 to Ulaanbaatar from Russia. I do not have a lot of vacation left and have 6-7 days in which I want to cover part of the Trans-Siberian rail and Trans Mongolian rail.

-I plan to fly into Novosibirsk and take the Rossiya(Rossija) to Irkutsk.

-A day and a half from Irkutsk to visit Lake Baikal. Would that be sufficient?

-Trans Mongolian rail from Irkutsk to UB. (As you must have already done this trip, is the Irkutsk-UB scenery similar to Novosibirsk-Irkutsk scenery by train?).

Q) Should I take a flight from Irkutsk to UB, instead of the train?
I understand that the train would be a lot more fun since I would get to meet people etc, but I am short on time and want to spend time exploring Baikal and some parts of Mongolia in the 7 days I have.