I'm having a really, really tough time deciding between these two lenses. I've been reading reviews for weeks and waiting for a sale. I am primarily a concert photographer, this lens will of course be used in low light situations and to capture wide shots of events, including crowd and stage. My greatest concern with Sigma in general is low light AF, and I understand the 20mm is better than the 24mm. The 24mm I realize will be slightly more versatile, yet people rave about the 20mm. The last bit of angst is in the weight of the lens and bulbous front - not such a big deal because I'm not likely to use filters.
Any comments, comparisons from anyone who has owned and/or used either of these?