thank you for all of your replies!
thanks =X= and murraypaul for informing me that 16:10 is the ratio that best matches the standard US comic ratio for a single page. That fact has definitely persuaded me that 16:10 is the ratio I should go for

i was quite surprised because I would have thought 4:3 would be the best match, seeing as it is much wider in portrait mode than 16:10
it's a relief because most android tablets use the 16:10 ratio, and videos also look better on this ratio than 4:3

the only downside would be that ebooks and magazines would probably be better on the 4:3 ratio, but you can't have everything!