I had both. Zony 35 f/2.8 is a true compact lens and feels really close to a pancake lens. Sigma 35mm f/2 is not big, but coming from Zony, it will feel much bigger and heavier. I was not happy with the IQ of Zony and Sigma 35/2 didn't really feel like a small lens to me, so I dumped both and got Sony 35 GM.
If you are looking for a compact lens, stick with Zony 35/2.8 or look at the new Sony 40 f/2.5G. If you are looking at the best 35mm, Sony 35 GM is your answer. No other 35mm does it all (size, price, weight, IQ, and rendering) like the 35 GM. If you don't have a budget for the GM, Sigma 35/2 is probably your best option. It does everything well and it feels like a premium product due to its high build quality and design.
