[EDIT: This is an old device, rebranded. Here's all you need to know about the Transporter.]
Stereophile's lab measurements of the Transporter and the ZP80 seem rather equal in terms of quality, so if you have a good outboard DAC, the sound quality of a Sonos ZP80 and a Transporter should be equal. I would think if you're doing analog out of the Transporter or the Sonos ZP80 (using their builtin DACs), the Transporter would win (at least would come closer to a "win" as silly as it is to talk about that). If I were making a pros/cons list, I'd say:
Transporter pros: world-class DAC, XLR connectors, beautiful form factor, ability to play WMA lossless files
Transporter cons: price
Sonos pros: great digital output, extensibility/ease of use, great controller, ability to stream analog audio to other Sonos devices via analog inputs, price, builtin amp option, creates its own wireless mesh
Sonos cons: clunky zoneplayer form factors, no WMA lossless support
Few cons, either way.