Which would you rather fly? A civilian Trans-U, or a civilian Firefly?
The Trans-U has a faster firing magnetic pulse weapon. The pulse itself is also a lot faster. The Trans-U has the same hull mounted AA gun the Firefly mounts. Both ships roughly are equally fast. Though the Trans-U is a lot less maneuverable. Both ships have about the same armor rating, but the Trans-U has about 30% more hull integrity. The Firefly has better Flux though.
The Firefly is smaller and more maneuverable than the Trans-U. Making it a smaller target. They are roughly of the same speed though. The Firefly can fire more magnetic pulses, thanks to higher Flux, though they are spread over a longer time period, since the Trans-U can fire more rapidly. The Trans-U's pulses also move faster. Both have the same hull mounted AA gun. The armor rating is roughly the same, though the Trans-U has about 30% more hull integrity.
To further illustrate the difference between a civilian Trans-U and a Reaver Trans-U:
A Reaver version would be slightly faster, thanks to its uncontained core, and lesser mass. It gets less mass because it lacks armor plating. This means the Reaver Trans-U has a bit less armor, but it does have equal hull integrity. Having an uncointained core means it needs less minimum crew to operate. It also means it has slightly less maximum crew, since the living conditions are lethal. Still, being originally a cargo vessel, it still offers reasonable space for a boarding crew. The ship also has easier Flux venting, thanks to its uncointained core. This means it can fire its weapons more often. It also has an extra weapon: a forward mounted harpoon.
In short:
-slightly faster
-less mass
-less armor
-less minimum crew required
-less maximum crew
-faster flux venting
-extra weapon