I've lived in the US and also in a country that used the A-series extensively. While I've always appreciated the beauty and math of the A-series, I find the aspect-ratio of the US Letter paper to be much more appealing. A4 tends to be too long and too narrow.
> US Letter: 216 mm x 279 mm
> A4: 210 mm × 297 mm
so it's 18mm longer while being 6mm narrower. At that small a difference, I would assume that a preference of one over the other is similar to that of metric vs. imperial - the one you were raised on just seems right.
Which one did you grow up with?