This is a tough question, because they certainly are interchangeable in many cases!
These are examples where they are not:
*電話で長話をしました。そうしたら、母に怒られました。(I was talking on the phone for a long time. Then, I got a scolding [from my mother].)
*そうすると、私が責任をとらなければいけないんですね? (So, that means I must take responsibility?)
The key difference is that そうしたら usually describes the past or present and そうすると is used to describe the future or a probability.
Sorry, I'm not a grammarian--can't give you an authoritative answer.