There are two categories for penile implants: the malleable (semi-rigid) and the inflatable (hydraulic). Both give excellent erections. The difference are: The malleable penile implant has two states, the erect state for intercourse, and the bent state for concealment. Wile the inflatable penile prosthesis has the two states of erect and flaccid, much like a normal penis. So it would seem that the inflatable is better than the malleable as regards concealment. This is true. However, the malleable are much lower in cost and are much less prone to mechanical failure than inflatables. Mechanical failure is rare in both cases, but even rarer in malleable. Click here for a full comparison between types of penile implants.