[Edit: Turns out the Fleet Advanced has 5 Tac consoles too, thanks, mrgrocer56!]
The Mobius is certainly better than the regular Defiant-R, but a FLEET Defiant-R...at that point it ALMOST comes down to build/gear/tactics. But...yes, the Mobius edges it out in the end, if only for reasons of a slightly higher shield modifier and the flexibility offered by the Mobius' universal boff slots. With a Uni Ltc and Uni Lt, you can do a lot of different things with a Mobius that the Fleet Defiant can't. Plus the Mobius gains extra perks for folks with a Wells and/or Temporal Set, so it has more gimmicks by default.
However, almost no other Fed ship turns or moves as fast as the Fleet Defiant, and no other ship in the game than the Bug has 5 tactical consoles (I think). And of course, no other Fed ship besides the Gal-X (which has no Fleet version) is compatible with a cloak either, even if it isn't a battle cloak. The Fleet Defiant has some terrifying First-Strike potential. In what few PVP matches I've played at the endgame levels I've seen skilled Defiant players MELT ships with 50-60 times the crew and supposed durability.
That said, it's almost a given that a lockbox ship would be expected to slightly outclass even a shipyard Fleet ship. In terms of the relative amount of money spent, a Mobius costs far, FAR more than any Fleet ship. At the maximum a Fleet Defiant will only cost $30 (Cost of C-store Tac-R + 1 Ship module, discounting whatever resources you dumped to get your Starbase to Tier 5). If you got your Mobius by Lobi alone you probably paid up to $200.
Thanks I have a fleet defiant and have my eye on the Mobius because it looks cool.