Hello there's a very similar thread concurred you being co tri used to. Here is that link...

http://www.bikeforums.net/commuting/1114642-panniers-vs-backpack-3.html#post19795494

My experiences have been with both a backpack as well as panniers. I prefer panniers. When I'm in school (I also work full time) , I commute with an ortlieb office bag 2.1. Fits my books,pens, papers, laptop and lunch. On the other side of the rack i use an ortlieb sportpacker plus for my change of clothes, rain gear and shoes for work. When I'm not in school, I use both sportpacker plus panniers to hold whatever I need for work. I keep all my tubes and multitool in a saddle bag and my pump is attached to the frame.

Hope that helps.