The difference would be pretty negligible. The defiant has a better console layout and can field a better shield tank, it also has the option to equip a cloak.. It makes the Fl Defiant far too powerful for a t3 fleet ship.
There is no correlation to ship power and tier they are located in, not at least one that makes any kind of rational sense. If anything it's completely arbitrary and cynically I think they are placed by perceived desire to push people to continue progressing in tiers.
Otherwise the Fleet Galaxy-R would not be T5 by any stretch of the imagination.
Nor would the Fleet Vor'cha, easily one of the best ships available KDF side: a ship with 8 weapon slots, DHCs, cloaking device, 10 base turn rate for a Cruiser - be T3.