I own the canon 24 and love it for live music but it firmly stays at home in favour of other lenses for anything astro related. rokinon/samyang 14mm is my go to along with Sony Zeiss 18mm.
I studied this question for a long time and bought the 14mm several years after the zoom. I really think the zoom is the way to go. Not sure the 14mm was a good purchase.
The Irix 15mm F/2.5 is an awesome lens for the price, ($430 USD). Far better than the Rokinon/Samyang offerings, and great for typical landscape work too. Might just be the only 15mm with filter threads too.