Remember that even if aluminum peaks, with overcoating, at 91% at 550nm, and silver might be 97%, that difference isn't going to really make a huge difference visually.
A difference of 0.1 magnitude (9.65%) isn't going to be decisive in the revealing of details in deep sky objects. This will affect, largely, only things at the very limit of the scope's ability to pick up photons,
and is likely to be in the realm of what percentage of the time a particular feature is visible to averted vision.

If you are truly pushing the limit to see something at the very limit of visibility (and I think very few ever really do that), then gaining a few percent in coatings and a few percent with simpler eyepieces might enter the picture as significant. A few here on this forum have seen small differences by doing so.

If, after a few years with the 18", you think that the extra few percent might help, then get the mirror recoated by someone like Terry Ostahowski with an enhanced aluminum coating.
Whatever coating you get won't last forever anyway.