From Canon T3i to Nex-6. Am I giving up some image quality?

I agree the build quality isn't the equal of an actual Zeiss 35mm lens but it is much better than the $200.00 Sigma 30mm.

Realistically speaking, Sony is not Zeiss and even though Sony/Zeiss lenses are manufactured to mimic Zeiss lenses, the NEX lenses are built as small and as light as possible. This alone would cause the lens to appear cheaper.

I'm not sure about the glass, which is one of the most important aspects of a lens. There are different quality levels and I know Zeiss uses some of the best in their native lenses.

The glass and the fact that Zeiss lenses are a German product are two of the reasons Zeiss lenses are expensive and, with a 35mm size lens, you have a lot of that glass.