I use a small 20L Arc'teryx backpack called a Spear 20 ... was about 90€ ... and I bought it in 2011 and have used it every single day since

https://www.google.co.uk/search?q=Ar...ed=0CAcQ_AUoAg

It has two shoulder straps, a chest strap and a waist strap. I don't wear the waist strap, I do wear the chest strap.

A 13.3." retina MBP fits securely in the water bolder area. The roll-top is nice for longer items like two larger prints that my student recently bought me.

I don't like panniers and they're bulky when going through train or airport security. A backpack is much better. Also, jumping of a bike and onto a bus/train is 10x easier with a backpack as than a pannier.

Hope that helps.