What are the pros and cons of panniers vs messenger bags?
A messenger bag is great when you:


Are not transporting a heavy load
Have to put items into the bag, or remove items from the bag very often.
Are doing short trips, or having to leave your bike a lot as part of a longer trip


A rucksack is great when you:


Wish to be able to easily carry the load while off the bike
You have to walk a long way after your bike ride, or use public transport
You wish to have both hands free when not the bike, e.g you need to carry the bike up stairs at the start/end of your trip


Panniers are great when:


You care more about the “on bike” part of the trip than the “off bike” part
A pannier can have a solder strap, but it still takes time to fit and remove the strap or tie it up so it does not catch in your wheels.
(You will need some sort of rack to fix the Panniers to)


A basket is great when:


You wish to transport a normal bag (or rucksack) on your bike
You like the fact your the bag can be removed quickly by you (and don’t mind that it can be removed quickly by others.
You think being “practical” is more important the being “cool”.


A luggage carrier / rack is great:


When you wish to fix a bag or box onto the back of your bike
Unlike a basket you need to use straps (sometimes build into the rack) to fix the item to the rack.
You are less likely to get laughed at with a rack then a basket.
So it takes longer to fix something onto a rack then put it into a basket, but also it is harder for someone else to remove the item.
(Panniers are fixed to a rack; this is covered under the pannier section)