From an edge to edge sharpness, the Zeiss 15mm wins hands down, compared to Canon's equivalents (excluding the TS range). The TS lenses from Canon are probably on par with the Zeiss. But, they are designed with a much larger image circle, to account for the shift that the lens can do (think of a FF lens on a small sensor).
For my testing (and I realise it's not perfect) I simply hired a few lenses and took some shots of the stars, which quickly sorted out which had coma or not (coma creates edge softness in a lens, not really correctable in PS or Lightroom). For example, the Canon 24mm f1.4 is a beaut lens, in the center, but soft around the edges. Point it towards the stars and you will see why
That said, it'll depend on what the lens is intended for. Are you wanting wide open usage, or can to stop the lens down a bit?