All things being equal, by equal I mean same rim profile and same tire, I believe that the 26" would accellerate quicker. Why do I think this? The weight should be the same (OK, the 26" could be slightly lighter since there is less rim circumference), but the fact that 26" rim has a smaller diameter is really important. A smaller diameter means a smaller moment of inertia. A smaller moment of inertia means less effort to spin it up.

The smaller one has to get to more RPM though, losing the benefit. For a 20mph road speed, the rim and tire have to get to 20mph (or very close to it) around the hub anyway, regardless of wheel size. The slightly smaller 26" size will give very slightly less wind resistance and slightly more rolling resistance, the latter being more of an issue at the slower touring speeds and heavier loads, and the fact that the wind resistance of the panniers and other luggage will somewhat dwarf that of the wheels. The difference in speed between the two sizes is totally negligible (again, all other factors being equal, especially between tires).

The myth of wheel weight (related to what paddleboy brought up) and acceleration has been soundly debunked many times though. For years, it was kind of like the Paul Bunyan tale which got taller every time it was told. Take a look at

this wheel-science web page. Even if that first equation is too heavy for you, you can look down at the charts and see that even cutting your wheel weight

*in half* (an absolutely

*huge* difference in wheel weight!) makes only a fraction of a percent difference in the power required to go a given speed. The difference made by just moving the weight to the middle so it doesn't count so much as "rotating weight" is even much smaller-- .004% and .008% in one case.