I'm tempted by the A6, but I know I tend to write more than I expect once I get going. For those who've used both, what made the A6 feel cramped, or the A5 worth the extra size?
Since you tend to write more once you get going, try a few typical entries on page-sized mockups at your usual handwriting size. If A6 regularly makes you squeeze lines or cut details, A5 may be worth carrying; occasional overflow alone probably isn't.
Stickers and little bits of paper can make A6 feel cramped even when your writing fits. If you want room for both writing and decorating, I'd lean A5.
Since you tend to write more, A5 is appealing, but where you'll use it matters too. If it'll come along every day, A6's smaller size may be worth the tighter page; if it mostly stays at home, A5 seems easier to justify.