Personally I've always found handlebar bags to compromise steering and handling, and they seem to always interfere with cable routing too.
So if you've not ridden with such weight on the front, it would be worth testing a backpack with some weight in it, strapped in place. If it feels weird, you might not want to carry your stuff out there.
The direction of the flap matters depending on how you want to access the content. If you will open it while riding, then a front-secured access makes sense. Same if you want to stand over your top tube. If you want to be in front of the bike to l…