Yeah, here in Bend, we have lots of potholes and rough roads, so it sounds like in addition to the better mileage, the 19” are the way to go. I just wanted to make sure I want missing something in the pros/cons.
It's not just 19s vs 21s, it's all season vs summer performance tires...two different factors in play.
First, 19 vs 21:
The 19s have taller sidewalls so there is more air cushion between the tread and the rim (or wheel) hopefully helping you avoid the destruction of your wheel even if the tire fails. When you hit irregular surfaces on the road, this air cushion helps absorb some of those irregularities creating a smoother ride. The taller sidewalls also means that there is flexing when turning or making evasive maneuvers. The 21s have shorter sidewalls compared to the 19s so less air to cushion irregularities so hitting a deep pothole is like hitting it with just your wheel. Shorter sidewalls means less flexing so better handling and responsiveness.
Then you have summer performance tires on the 21s and all season on the 19s. Let's assume that Lucid has specified high performance all season tires.
Summer performance tires generally have shallow treads with small gaps between treads and little to no siping to channel water/snow away. They are made with a soft compound so they grip better for better acceleration and cornering. The small gap between treads provides more surface area contacting the pavement. Soft compound with shallow tread translates to short life.
Pros:
Good handling, better acceleration, feel the road more, generally more stable at high speeds.
Cons:
Short life, might not handle wet conditions well and definitely not snow, feel the road more. Not recommended in temps below 40.
All season tires are an attempt to blend characteristics of a summer tire with a winter tire. Deeper treads, harder compound while still attempting to provide good grip. Wider gaps between treads and siping help channel water/snow away from tire. Some all seasons lean more toward dry performance while others lean toward wet. You might think of them as "jack of all trades, master of none". So a high performance all season will give you some of the performance benefits of a summer tire but not as well and it will give you some of the benefits of a winter tire but not as well as a tire dedicated for that purpose.
Pros:
All purpose tire which means it fits more drivers than dedicated purpose tires and for many, it's the only tire they need. Longer life than summer tires. A well designed one will perform well in dry and wet conditions. Better range due to lower rolling resistance compound.
Cons:
Doesn't perform as well in grip and cornering/braking as summer tires. Doesn't work as well in snow/ice as winter tires.
One strange note is that insideevs lists the GT with 19s as faster 0-60 than with the 21s while both DEs are faster with 21s than 19s. This could reflect what hmp10 pointed out - only the DE 21s are listed as truly forged wheels so maybe the GT 21s are heavier.