Again, depends on the airline.

It's certainly safer to book the later flight. If it turns out that all flights are sold out / overbooked / or if there are weather cancellations, then at least you're guaranteed to get home on your original flight. Book and then miss the early one, if the later ones have no open seat, you might be plain out of luck.

If you do decide to risk booking the early one, and can't make it, make certain to call the airline before its scheduled departure, or they may cancel your ticket altogether.