The Phillips 3-4 is a one gap defense, meaning each player in the front seven is responsible for only one gap. The scheme is based on penetration, speed, and attacking the passer. The only positions that really hold up blockers are the DEs, which are supposed to occupy space and blockers to give the OLBs a free shot at the quarterback. Think of them as the OLB's "fullback".
Parcells' 3-4 is a two gap defense, which means each d-lineman is responsible for holding up two gaps and not penetrate. The only real pass rushers in a Parcells 3-4 are the OLB, where Wade's can come from anywhere. Like someone said before, Parcells is read and react, play it safe, hold position. Wade likes to take risks and attack.
Both can be successful if played with the right players, but as we saw in 2006, Parcells' scheme was way too vanilla and had become easily figured out. Other coaches that run the same type of 3-4, like Belichek at one time and what they run down in Miami, have adapted to the times with the same scheme.