I would personally prefer the Sentinel class from Rebels, only because we haven’t seen it in the films before, and it is more clearly a transport while still looking similar enough to the original Tydirium to not feel entirely alien. The zeta class would be too radical a change IMO, and it’s role is so similar to Rogue One that it would feel like a repeat.
For reference:
Sentinel Class:
http://vignette1.wikia.nocookie.net/starwarsrebels/images/4/44/Sentinel-classShuttle.jpg/revision/latest?cb=20141125200724
