i tried with a backpack for a while. it wasn't bad. other than my back getting sweaty i have no complaints. even in the rain, as long as i had a rain cover for my backpack (it was included with the backpack) it was more or less fine. out of an abundance of caution, i used a waterproof stuff sack inside my backpack to ensure that my clothes would remain dry. the only reason i went with a rack and panniers is out of curiosity. i went with an axiom streamliner disc dlx and ortlieb sportpacker plus panniers. in my opinion, the rack and pannier set up works great and they serve my purposes well. admittedly they're not as agile as wearing the backpack in while riding but i kinda enjoy riding the bike unencumbered by the straps of the backpack. also, because i use both panniers, i'm able to keep one loaded with things like rain gear, my u-lock and throw my lunch inside of it and then use the other pannier to hold my clothes and things that i can just easily take off the bike and carry in with me to work. whereas carrying all of those things on the backpack was possible, it isn't as easy as carrying all of those things via the panniers.

my vote is rack and panniers for convenience only.