It seems Canon has developed different coating technologies for different applications instead of trying to force a "one size fits all" solution on all of their premium lenses.

As far as I can tell, Canon does not use both ASC (first released with the EF 100-400mm f/4.5-6.3L IS II in 2014) and SWC (first released with the EF 24mm f/1.4L II in 2008) on the same lens elements. It's either one or the other.

• ASC seems to be used for fairly long focal length lenses
• SWC shows up in wider focal length lenses

As with many things related to lens design, what is "better" than something else depends upon how it is being applied and what one intends to do with it. Canon seems to have decided that ASC works better (or works just as well at a reduced cost) for lenses with narrower angles of view while SWC works better for lenses with wider angles of view. There's also a hint (included in the quotes below) that ASC might not work as well for lens surfaces with significant curvature, while SWC can be especially effective when applied to lens elements with significant curvature.

The following quotes are from Canon's article Evolution of "L"

Regarding ASC:

Air Sphere Coating (ASC) technology creates a film containing microspheres of air over a vapor deposition lens coating. Systematic lining of the inside of the coating with spheres of air forms an ultra-low refractive index layer. This results in extreme anti-reflective performance, particularly for incidental light that enters at nearly a vertical angle, effectively preventing flare and ghosting. The layer of air microspheres is covered by an interfacial layer, so the ASC coating is highly durable.

Regarding SWC:

Anti-reflective Subwavelength Structure Coating (SWC) suppresses light reflection with countless wedge-shaped structures more minute than the wavelength of visible light on the surface of a camera lens. This coating layer enables continuous variation of the refractive index, resulting in full-scale suppression of reflection. SWC realizes excellent anti-reflective effect even with a large angle of incidence, and dramatically reduces flare and ghosting in peripheral areas of the lens, which were difficult to suppress with conventional coatings.

Comparison of the two technologies from Canon's article ASC: Reduction of Flare and Ghosting

ASC utilizes the anti-reflection principle from destructive interference of light, and it has excellent anti-reflective effects particularly with respect to incident light that enters almost vertically. However, even with increased angle of incidence, ASC still can significantly reduces flare and ghosting that cannot be prevented with conventional vapor-deposited multi-layer coatings. ASC can also be applied to a variety of lenses, making it extremely versatile.

The Subwavelength Structure Coating (SWC), on the other hand, applies a different anti-reflective principle. The advanced Canon developed technology uses a special process to arrange a countless number of nano-scaled (200-400nm) pyramidal structures, smaller than the wave length of visible light, on the lens surface. Since there are no surface boundaries where refraction indices change significantly, making it possible to prevent the reflection of light by implementing gradual refractive index modifications between air and lens. SWC has excellent angle of incidence characteristics, exhibiting excellent anti-reflective effects even for light with particularly large angles of incidence, even better than ASC. As SWC can also applied to surfaces with significant curvature, it is now possible to reduce ghosting and flare that can occur around the periphery of even lens elements with a large curvature in wide angle lenses.

From these descriptions, it sounds like SWC may be more expensive to implement but is also more effective with lenses that have surfaces with a lot of curvature. ASC, on the other hand, sounds like it is cheaper to do but works just as well (or even better) with lenses where most of the light entering them is at lower angles with respect to the lens' optical axis.

At the end of the same article is this summary:

From the initial design stage of a lens’ optical design, the importance of reducing flaring and ghosting has already been taken into consideration. Base on the lens types and specifications, Canon engineers utilize SWC or ASC on the most appropriate lens surface to reduce flare and ghosting in order to raise image quality. This is same as the utilization of fluorite, UD glass, ground aspherical lens or glass-molded aspherical lens, consideration is made for the total performance by using the right element in the right place.