Another option, though a little more work, would be to create new iCloud accounts for both you and your wife but continue sharing the same AppleID for app/music purchases. That way there's no need to buy your content all over again and each of you can have all your stuff backed up to iCloud in case you get a new iPhone or something. Again, it'll be a little more work than just turning off Notes in the iCloud settings, but still something to consider.