Its my understanding that forged metal is on average, more durable. However, casting done correctly has the potential to be stronger. For example, Browning switched from forged frames on their High Power to cast frames, in order to accommodate higher powered rounds like the .40 S&W.
Thus, it is hard to make a decision based solely on forged vs. cast, if durability is the criteria. You also have to take into account the reputation of the company and its products in order to "triangulate" how durable their firearms are (kinda sounds axiomatic though).