I generally dislike carrying much in backpacks, hate one strap bags (sliding around is what they do, especially if you get out of the saddle to go uphill) and love riding with the weight in front LowRider racks and panniers. And there are very good, fully waterproof panniers. (This was not true when I started cycling, but Ortlieb did the cycling world a huge favor by showing how it could be done. Now many others, inspired by them, make good waterproof bags and panniers. I still use the Ortliebs. The pair I bout 19 years ago hasn't worn out yet and the 9 year old pair look almost new.

I believe Ortleib now makes and I know some of their competitors make panniers with shoulder straps so you can carry them on the bike, then lift them off and sling over your shoulder when you lock the bike. My older Ortleibs have carry straps and I actually prefer to carry my dirty bags off the bike on wet days to throwing them over my shoulder.

Ben