I have the 24 ART. Simply put, it is excellent. And somewhat surprisingly is shows excellent bokeh wide open. It also is very low distortion. Corners are softish wide open, clean up as you stop down to f4 where it gets very good. At f5.6 the entire image corner to corner is as good as I've seen from a 24.
I do not own the ART zoom, so cannot comment other than my impression from what I've read and seen. It is also excellent, and especially so for a zoom. The MTFs indicate it is very close to the 24 prime at comparable apertures, which is remarkable. I do not know about distortion, but assume it has more than the prime, simply because of the nature of zoom design.
So, it comes down to comparing one of the best and fastest 24 primes available to a lens that is a stop slower, $150 more expensive, 1/3rd heavier and a little longer. But for all those negatives, you get zoom convenience and cover three major focals at near prime performance all in one lens...
Not an easy decision...