With LifeAlert, you basically yell into a base-station (speaker box). Hopefully, the individual is within speaking distance otherwise they won't be heard. With Medipendant, the pendant is a two-way communicator. Thus, you can speak and listen through the pendant. A huge plus.
Also, there are no long-term contracts with Medipendant (not so with LifeAlert). Lots of horror stories about LifeAlert on-line about their contracts, etc.
The Medipendant is just a better device all around for those that need it to give you the peace of mind that your parents/grandparents have someone to turn to incase of an emergency.