Cast and extruded acrylic are manufactured in different ways using the same components. Either will be acceptable in many use cases, however there are some important differences between the two.
Optical clarity and surface quality
Cast acrylic has better optical clarity and surface quality. It is also harder which makes it slightly more scratch resistant.
Fabrication
Cast acrylic will cut and machine more cleanly and is easier to glue. When used for thermoforming, extruded acrylic will expand or shrink differently relative to the extrusion direction which can make bending more complicated. Extruded acrylic is easier to flame polish because of a lower forming temperature.
Laser cutting and engraving
At the same speed and power level, cast acrylic will give cleaner edges, which makes it easier to cut. Adding a little more power will give extruded a smoother edge as if it was flame polished. Cast turns matte white when engraved which can make the engraving stand out more from the acrylic depending on the color. Extruded does not change color when engraved.
Thickness tolerance
Extruded acrylic has better thickness tolerances – usually +/- 5% or better across the sheet, while cast acrylic can be +/- 10% or more. This can cause issues in certain cases where the tolerance is important, such as installing the sheet into a pre-existing frame, or a precise model where many pieces need to fit together.