Both can cause a lowering in ride height due to age. Springs take a long time to "sag" if you want to label it as such. They do not lose rate, they simple loose free height over time. The bigger issue is shock gas pressure. Shocks will cause a loss in ride height because the shocks will loose gas pressure in time. Especially your average everyday OEM shocks. They usually go bad in about 5-7 years. Buying just new shocks would be your best option. It you want to maintain more of an OEM ride but with a bit more sport feel then I would highly recommend you buy some high end Bilstein shocks. These are gas charged and will give you back I would guess about 1/2" ride height maybe even a little more. A Koni will do the same (Konis are my favorite) but Konis will give you much more a performance sports car feel. The Koni;s have more rebound force overall (THis is adjustable) and also have a slightly higher compression force then the Bilsteins. Both of the above mentioned will give you about the same ride height change over the shocks you currently have.

I would not recommend bothering to change your springs. They should be fine. Just change shocks.