According to When to Ally, When to Acquire:
a) When the ratio of soft to hard resources is high, an acquisition would be more appropriate so that the soft resources do not leave the company.
b) When the resources are similar in both the target and acquiring firms, an acquisition would be more appropriate so that there is more control over the resources.
c) When the level of competition for a target company is high, an alliance would be more appropriate so as not to overpay for the target.
d) all of the above