For practical purposes, there is no difference between the performance of Standard Load and Extra Load tires - except for load carrying capacity (which occurs in XL tires at a higher inflation pressure).
HOWEVER, there are such huge differences between make/model of tires that many people THINK there is a difference between an SL and an XL. They are comparing apples and bananas.
Also, do NOT confuse XL tires with LT tires - which come in Load Ranges (C, D, E, etc) LT tires are a different kettle of fish from both SL and XL Passenger Car tires.