Aerated or heavy concrete block?

I'm about to replace the roof on an old large garage made from 'medium density' concrete blocks. On my neighbours side the top 10 inches or so is made up in very rough brick and looks bad from his side.
We agreed I replace this top row in blocks before adding the new roof, but does anyone know if the modern aerated blocks are suitable for exposure to rain, about half a block will show under the new metal corner sections for the roof? In other words, will the aerated block wick moisture through to inside my garage enough to be a problem, or are they the same as the heavier ones?
TIA Mike H
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