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.
Sidewall stiffness is really a major factor in the ride quality. Next time you go to a tire store, check the sidewall stiffness. You may find significant differences between brands. Performance tires often have very stiff sidewalls and have an equally "taught" ride.
I usually opt for the XL tires. It goes up to 51 PSI.
Ah ..... Mmmmm ..... Not exactly.
Barry's Tire Tech: Load Tables
Short version: SL tires are rated at 35 psi, but it is permissible to mark the max pressure as 35, 44, or 51 psi. This has to do with the speed rating.
But XL tires are rated at 41 psi - but isn't a specified way to handle those higher pressures for higher speed ratings, so some tire manufacturers default to the 44 or 51 psi, and some default to 50 psi (+6 psi over the 44 psi value. I've never seen anyone use 59 psi!)
So on XL tires you may find 41, 44, 50, or 51 psi as the max pressure.
I find my ride to be stiff even though I have new bushes and new shock absorbers.
I run Pirelli P7 Cinturato tires which are Extra Load.
I am not sure if the tires are at fault here or not but I may go a different tire next time (even though I am very happy with these overall)
The cinturato p7 (summer, not all season) I had on my car when it came from the factory had very soft sidewalls, The crossclimates I use during winter months have very soft sidewalls aswell. Ride still feels firm over irregularities.
You will get a slightly more stable ride when fully loaded with less rear end wandering. I am replacing an old set of Cooper extra load tires with regular load because of costs for my older truck. The XL tires only lasted around 40-45k miles. Nothing to get excited over.Tread and sidewall plies are the same between the two and about an extra 200 lbs weight carrying capacity per tire.