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): Oatmeal Stout, Hard Cider, Belgian Tripel, Liquid Stupid, Belgian Dark Strong, Red Bull, Rye IPA, Double IPA, Imperial Stout, Irish Red, Scottish 70/-, Belgian Wit, Nut Brown, Traditional Mead
Walk-In Cold Storage(~50): Cysers, Braggot, Melomels, Ciders, Traditional Meads, Persimmon wines, Dry Stout, barleywine, cream ale, sodas, Chocolate stout, milk stout, and others I can't recall.
Fermenters (0): Nothing current.