I have owned both. I bought 15mm limited first and it was quite good. Size, filter use, starburst and center sharpness were the positive things for me. The biggest negative thing was sharpness in edges and corners (i think some were due to field curvature), why I sold it and bought Samyang 14mm.
Samyang (first copy) was not sharp on the left side, so I got new one. This one is way better than first one. I calibrated the focus ring and now focusing is easy. I think that Samyang is sharper and have same contrast and color than da15 limited. Only the extreme corner and extreme edge is soft in certain circumstance. Only negative thing is lack of filter use (there is way to do it, but it's not easy or cheap).
If you want to see samples with Samyang 14mm + K5 (there are also other photos taken with another lens but you'll notice it):
Hikes and trips - Vaellukset ja reissut - samsshots
It worked well with K5, now i have K3 and still good. Great lens.
Sami