I've tried both and I would say that they are rather different. Saaz is going to be lower AA, and spicy. Sterling is a little higher AA and has a much more perceptable floral character - maybe even slightly citrus-y, though nothing like cascade.
Both are clean aroma/flavor hops, and good choices either way. I'm planning on ordering both myself, though I'd say that Sterling is probably a little more versatile as it could work well in alomost any style.