Commuting with a Rucksack: Change to Panniers?

[Furton]

For the last month or so I've been commuting to my local station (2.5m each way) on my Giant Defy 4. I go in my work clothes as it's not a difficult ride and don't get sweaty. However, my back does seem to get quite hot and sticky as I carry a rucksack to work.

I've been looking at rear bike racks and can't find a definative answer to my question. Am I better off strapping my existing rucksack to a rear bike rack with bungee cords, or should I invest in some panniers which I could put my rucksack (20l) in?

 

[Furton]

Thanks for the suggestions. I'm going to need something I can quickly take my rucksack out of at the station, so ideally a large waterproof bag!

Is there much difference between rear bike racks other than weight when you spend more?