Ok you seem to be really struggling with the entire cycling think but here goes.
Backpacks are OK to cycle with but not ideal. I've been using a 32L capacity LoweAlpine knock-off brought in Cambodia for $10 more than 2 years. Its fine for a commuting with clothes and shoes to and from work, sometimes I find it’s a bit small but that is mainly due to having massive (Size 14) shoes which take up half the bag. Pain in the arse to get stuff out of like keys and security pass to get out of the bike lock-up at work. The zips are starting to go and some of the inner pockets are tearing out, but overall happy with 2 years service from a bag I picked up for $10 in Cambodia. When I mountian bike I have a Camelback backpack but it only carries a 3l bladder and some tools and spares.
I’m on the list for one of Scott’s big Theo bags because I want something larger and with greater flexibility on the bike and that will last a long time. Messenger bags are very comfortable to ride with and when worn correctly should spread the weight evenly over your back and have more comfortable strap positioning than a backpack. They are also designed to allow easy access without removing the bag, which is not the sort of feature you really appreciate until you have it.
Panniers are great for touring or long distance rides when you need to carry a lot of gear and are not on and off the bike heaps. They are good because the weight is carried on the bike not on your back which makes a big difference to wrists, back and arse after a long day in the saddle. However having loaded panniers on a bike make a big difference to the handling and you will really notice it if you’re riding fixed and have to get out of the saddle to get up a hill. They are also a pain in the arse to take on and off if you are not spending all day just riding along (i.e touring).
But what about back sweat?