GuideRails adds support on both sides of the heel to catch excess lateral and medial movement. It is designed to intervene only when the foot moves beyond its natural range. The Guide's medial post is firmer foam on the inner midsole that provides consistent medial support throughout the gait cycle.
Many runners find GuideRails less intrusive — it does not change the feel of normal strides, only intervenes when needed. Others prefer the consistent medial support of a traditional post.
Brooks Adrenaline runs slightly wider than Saucony Guide at standard width. If width is a consideration, the Adrenaline is the more accommodating option. Both brands offer wider fittings on their stability models.
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Source: specifications from saucony.com and brooksrunning.com.