I have not taken either tour but just have couple of comments:
1) You may want to google the distance between cities and try to figure out how much time you going to be on the motor coach. In Europe, I believe motor coach doesn't drive over 90 Km/H.
2) Review the location of the hotels at different travel sites. E.g.
www.tripadvisor.com. . One way for Cosmos to keep the price low is by using hotels not in the cental area or major tourist sights. This makes it hard to do/shop things on your own.
3) Check out similar tours by Globus (sister company of Cosmos) and Trafalgar, the hotels they use are supposed to be more centrally located.
4) Consider ending the tour in Paris and take the Eurostar back to London on your own. Why spend a day taking the coach to Calais, ferry to Dover, coach back to London, when you can go from Paris to Londer in 2.5 hrs? By ending the tour 1 day early, you should saved enough for the Eurostar and more time in Paris.
Just my 2 cnets.