For outdoor and landscape work like I do up in the mtns the 24 IS might just be the perfect ticket of size portability and performance. Not to mention it's almost half the cost.In that case, get the older 24mm for Canon at half the price of the new one. The IS isn't going to allow you to blur water and keep the images sharp at the same time, so you're either taking a tripod or shooting fast enough shutter speeds that the original 24mm is fine. Cause the MTF numbers that Lensrental showed basically say the difference between old and new is maybe noticeable on a print at f/2.8, but probably not. And if you're shooting landscapes, you're probably at f/8+ anyway, so no advantage gained.
For that one shot you need to take a 1/15 f/2.8, it's great, otherwise their is a much cheaper option that's just as good.