I've just ordered 4 Schwalbe Marathons: three of the 26 x 1.9 and a 1.75 Slick. And I've just finished some rolling resistance tests using my old tyres because I want to compare them to the Marathons when they arrive. I choose a day when there is no wind, ride part way up a small hill to a fixed starting point, then roll down hill about 200 metres till I stop. I do three runs to make sure the end point is repeatable. It turns out that it is, typically to within 3 metres.
When I lowered the air pressure from 50 psi to 30 psi, the distance I rolled decreased by 25m. ie about 12%. So I assume rolling resistance went up by 12% with the lower pressure.
My next series of test will be with the Marathons, and if I get keen I'll compare 1.75s with 1.9s at the same pressure but I don't think I'll see any difference.
If it is true that at the same pressure a wider tyre rolls easier than a narrow one, then to obtain the same rolling resistance with a narrow tyre means the pressure must be increased. So there comes into play another reason why a wide tyre will have better rolling resistance - and at the same time be more comfortable - the lower pressure in the wider tyres absorbs road shocks which would otherwise cause the rider to bounce up and down. Every time a rider is pushed up because of a bump, the bike slows down. The energy to push the rider up has to come from somewhere, and that somewhere is the forward speed. The effect is the same as when going over a speed bump: your speed slows as you approach the crest, then increases as you go down the other side; but you never fully make up the speed you lost. A wider tyre causes the rider to move up and down less (the tyre acts as a shock absorber and does the moving instead) so less forward energy is lost.
So, if the theory is correct, you can run a wider tyre at a lower pressure to have the same rolling resistance. You'll also get better suspension which of itself improves the rolling resistance even further.
But I suspect the difference between 1.75s and 1.9s would not be noticed in practice, in part because 1.9s are slightly heavier (by 90 grams), needing more energy to accelerate and to lift over hills.