iostream vs. stdio

so, I've got EXPERT PROGRAMMERS telling me to use printf instead of cout in c++, ok but why? what is the difference (in b4 stream)

>>31
not if you have a 32 bit environment and needs to do 64 bit arithmetic somehow, then cout could already display it, and I dont think the commitee added a int64 modifier to printf, %ld is long %lld is long long?