On a loaded tourer I think you have a choice between the Panaracer Pasela TG for spirited not too heavily loaded riding in not too demanding conditions (the Marathon Racers are similar, but a bit slower and a bit sturdier). I love the Pasela's for that, and have used them with great success. They are fast and comfortable, but not bombproof. If conditions are more demanding the ordinary Marathon is a great tyre. It copes well with gravel roads, it is sturdy, can carry heavy loads, and it is pretty puncture proof. Unless you are making a habit of riding in thorny or glass littered environments you really do not need the extra offered by the Marathon Plus, and you don't want the harsh and slow ride.
Much therefore depends on conditions. I have just come back from a rough and partly off road tour in Norway, and for that I fitted wide Marathon Extremes. I liked the extra grip offered by these knobbly tyres, and I liked the very low weight (there was a lot of hard climbing). The downside was more rolling resistance on tarmac, but those were the easy parts in any case. These are not general purpose tyres like the ordinary Marathons, but they fited my purpose very well. As soon as I have time I will put the Pasela's back on, however.
Finally, I would strongly advise to fit the widest tyre that will fit your frame. They give far more comfort, better grip on gravel roads, and are faster in real life conditions. The preference for narrow tyres is largely a remnant of the days when it was technically impossible to build wide tyres with light and flexible casings that were strong enough to handle the increased forces exerted by large air volumes. Technology has moved on, and there is now a choice of light and flexible wide tyres.
Willem