I'm trying to keep a small record of ordinary weeks, and these two approaches ask different things of me. Writing-only gets a thought down quickly; adding a bit of colour or something I've kept can make the page more memorable, but takes extra time. I'm leaning towards plain pages unless there's something I actually want to stick in.
Maybe keep the weekly pages for writing and put the occasional sticker or ticket on the monthly page instead? You'd still have somewhere for the little bits you want to keep, without having to decorate every week.
If you use the monthly page for a keepsake, add the week or date beside it. Otherwise, it can be hard to tell which week it belongs to later.