Being a tightwad I try to find the best deal possible and it seems that the Carradice ones are not only cheaper but also larger by 8 liters each, and as a bonus it weighs a tad less.
But right now the rear Ortlieb panniers are on sale at Bike Tires Direct which makes them cheaper than the Carradice:
https://www.biketiresdirect.com/prod...ier-set?sg=503 if you want the Ortliebs.