Whey concentrate has more lactose than whey isolate, and has less "pure" protein. This, in theory, could cause slightly more fat gain than whey isolate. It probably won't have a negative impact on muscle growth though, and some might argue the reverse (due to some of the other components in the concentrate form that are removed in the isolate form.)
If your primary goal is muscle gain, whey concentrate is fine. If your primary goal is weight loss, and you are using a diet plan (like 4HB's SCD) that restricts dairy, you might want to stick with whey isolate only. And of course if you are lactose intolerant, go with isolate.
6-1, 41 y/o male
SCD Jan 2-Feb 13 '11 (BW: ~193)
Modified SCD start Feb 13 '11(BW: ~185)
7/2/11: 179lb, 10.3% BF (based on hydrostatic test)
10/20/11: 185lb, ~10.5% BF
Dec '12: 194lb, ~11-12% BF (bulking phase)