US Letter or A4 PDF - which is better for iPad?
Letter.  A4 is slightly longer and narrower than US Letter, and would result in wider borders at the side to fit a full page vertically.  US Letter won't fit perfectly, but the aspect ratio of it's dimensions are a much closer fit to the iPads 4:3 (or 3:4 in portrait) screen and the slimmer borders top and bottom would be less noticeable than the A4 equivalent:

        Millimetres Inches   

        Width   Length  Width   Length
A4      210.0   297.0   8.26    11.69 (3:4.24 APPROX)


or 6% too long when scaled to fit the shortest edge, resulting in vertical borders when scaled to fit the longest edge

Letter  215.9   279.4   8.50    11.00 (3:3.88 APPROX)


or 3% too short when scaled to fit the shortest edge, resulting in horizontal borders when scaled to fit the longest edge

See this link for good comparative info and this one for a ratio calculator