When choosing Marathon Plus be aware that you are talking extra pounds not extra ounces. Also be aware that the sidewalls are quite stiff. All that may or may not matter to you. For me they were quite unpleasant, both due to the weight and the ride feel. Others love them. Just be aware of what the differences are and whether they matter to you. I found them unpleasant enough that I took them off and sold them.
My preference for the kind of riding you specify would be something like the kevlar bead 32mm continental gatorskin. Depending on which size Marathon Plus you chose what bead style and what tubes. The difference for two tires and tubes will at least three pounds and maybe four pounds or more.
Some people don't find the lack of a supple sidewall to be a big deal but for me it was a real deal breaker even when I decided to bite the bullet on the weight.
I have since become more and more weight conscious in my packing style so I now feel like it makes more and more sense to ride a lighter bike setup. I rode a road bike setup on the Southern Tier starting out with 23mm gatorskins because they were on the bike and I decided to wear them out before replacing them. I put 25mm gatorskins on in Texas and they were much nicer on the TX chipseal, but I really didn't want for wider. I was carrying only a base load of 14 pounds of ultralight backpacking gear though. I'd run at least 28mm and maybe 32mm for heavier touring.