Sigma Art 20mm:
Clearly sharper, especially on the wide open end (LensTip believes the 24L II is sharper in the center when stopped down, but TDP's sample disagree with that)
Less vignetting
Slightly better coma wide open for astro (see at LensTip
here and
here), but neither truly excel here
Greater ability to tune the AF on the Sigma USB Dock than with AFMA
Canon 24L II:
Weather sealed
More
consistent AF (not thoroughly proven yet, but assumed based on the Sigma 35 & 50 Art AF performances)
Better build quality (Presuming again -- I've shot the 35 Art and 24 L II, but not the 20 Art, I'm assuming it's similarly 'Art-Like'
Presumably better resale value
Personally? For me, the wider these Art lenses get, the less I need AF with them. At 35mm, the AF has got to be there for what I shoot, so I passed on the Art due to too many misses shooting wider than f/2 --> I'd get the 35 f/2 IS USM or 35L II depending on your budgets and needs.
But down around 20-24mm, perhaps an occasionally inconsistent AF won't kill you and the comically good value proposition of a super sharp 20mm prime is worth it. And let's face it, unlike Canon with two very good 35 primes, at 20mm Canon forces you to pricey and not a sharp primes in the 17-24 neighborhood (some lacking AF altogether) or forces you to a zoom. So hats off to Sigma for swinging for the fences here.
...except for astro folks, who may have thrown this lens out of consideration solely for coma reasons.
- A