Roissy bus vs. RER B

Hi everyone. My wife and I have been to Paris a couple of times but we have never flown out of CDG. In May, we will be staying a couple minutes walk from the Ecoile Militaire Metro stop. We are very comfortable using public transport, so a cab ride to the airport is a no-go. We could take the 8 to Opera, walk across the street and grab the Roissy bus. Or we could take the 8 to the 4 at Strasbourg St Denis and then on to Gare Du Nord to catch the RER B. We will leave for the airport at about 6:30 a.m. on a Saturday and probably flying out of Terminal 2E. Question: How far a walk is it from the RER B station stop to Terminal 2E? Question: Considering it will be an early Saturday morning trip, if we take Roissy bus, will traffic to the airport be reasonable? Full disclosure: I am leaning toward the bus, but maybe I am looking for some reassurance that that would be the right move from those of you who have used the bus.