The simple answer is nylon is light whereas poly-extra is heavy, but deciding which is better for your application get a bit more complicated. Use the following rules to determine which material you need:
1. WIND TOLERANCE
POLY-EXTRA is made to withstand persistent high winds, such as on a lakefront or on top of a hill. A Poly-Extra flag in a low wind area will NOT catch the wind well, which makes the flag less visible.
NYLON is made to withstand occasional high winds, such as in a neighborhood, a valley, or where there are tall buildings to block the wind. A Nylon flag in a low wind area will make the flag easily visible in high winds.
POLY-EXTRA is a heavier, more absorbent material that will not tend to fly well in wet weather conditions, however, this material holds up better in winter conditions.
NYLON is a lighter-weight, moisture-wicking material that can maintain its flyability is wet weather conditions. The downside of the lighter material is that it is more vulnerable to sun-fading and may rip more quickly in winter conditions.
BOTH POLY-EXTRA and NYLON are made with the same weave types and sturdy outdoor-ready threads. That is to say that they are both made to last in their respective outdoor environments; poly-extra is not necessarily better than nylon even though it tends to be more expensive. Each material has its own pros and cons.