There was a question a while ago on the old board about whey concentrates vs whey isolate.
I got to it too late i guess.Besides the speed of absorbtion, there are other differences between the two, making concentrates the better and healthier choice.
The concentrates have more lactoferrin(anabolic and immune system booster - mother's milk contains about 15% lactoferrin).Though you might find an isolate that has added lactoferrin(but costs more).Also, concentrates have more alfa-lactalbumin and less beta-lactalbumin(both being types of protein- human milk contains very little beta-lactalbumin).
Isolates have about 70% beta-lactalbumin, wich is allergenic( well it may not be a problem for most people, but in case you can't tolerate the whey isolates, this may be the reason).
Bottomline- just use whey concentrates, they're cheaper, have more fat(which is a _good_ thing).
Isolates do solve some digestive issues for people though. Overall, concentrates are the way to go.