Dave,
Assuming your current shocks are Bilsteins, I would stay with them, especially for the rear. Konis have a great reputation for ride and control but cost about 75% more. IMHO you won't notice the difference in the rear in any case. I just switched from Bilsteins to Konis on the front of our Marquis. While they did seem to give a slightly better ride and control, I would have a real problem justifying the cost adder again. The improvement isn't anywhere near the added cost so I'm sticking with Bilsteins in the future. Regardless of which shock you decide on, Shock Warehouse seems to have good prices and free shipping.
Steve