my SS and copper immersion chillers work great. i know all the numbers for cooling and thermal conductivity, but i'm willing to sacrifice 4-5 minutes of my time for the SS immersion chiller to cool versus the copper. The inability of being able to see inside my old copper IC was enough for me to switch to SS. It is a personal choice, really. Copper works better, but there are cleaning issues, and SS takes insignificantly longer to cool (no biggie), but i know i can get it perfectly clean every time, inside and out! If you choose to use a copper one, great! - I used one for a long time, but i got a deal on a 50' coil at McMaster Carr, and snagged it.
As far as the cooling times, my experience is that as long as i have a good whirlpool in my boil kettle (using my pump), my cooling is pretty much the same, plus or minus a couple minutes. Yes, copper is faster, but for the 5 gal batches i do, the differences are nil.
Look at the big breweries and large biotechnology companies and see what they use for heat exchanging (not plate heat exchangers, the exchangers in the tanks)- it is always stainless. Ask yourself why this is? Even if copper was that much better (25X) and there was NO worry of cleaning or other issues with copper, then we would see a lot more copper in the kettles and/or tanks. The fact is that SS is always used in these situations after careful considerations of all of these factors.
Bottom line, either is fine.