In my use of a pre-chiller, I wasn't impressed. I had to keep the pre-chiller moving in the ice water for it to chill the incoming water.
Now, I just recirculate ice water after using ground water to remove the bulk of the energy. I find this a much more efficient use of ice, and it works better for my setup.
If you are insistent on a double chiller though, I believe a post-chiller can be more efficient than a pre-chiller.
Whatever you decide to do, let us know your results!
Walk-In Serving(14): Flanders red, Hard Cider, Belgian Tripel, Liquid Stupid, Wee Heavy, American Brown, Rye IPA, American Amber, Imperial Stout, Dry Stout, Cream Ale, Belgian Wit, Traditional Mead, Oatmeal Stout
Walk-In Cold Storage(~50): lots of beers, meads, and ciders
Oak Barrel: Flanders Solera