In faction pure or ship pure, I kinda like the
ISS Defiant better. The MU have better captains/crew to make it a wonderful "Pocket Battleship". Step out of "Pure" builds, and they are both mean little ships.
There is a huge difference between 3 attack dice and 4 attack dice. The rule of thumb is that you want to roll 1/3 more evade dice then attack dice to have about even chances of evading the attack. If your opponent is rolling 3 attack dice, if you roll 4 evade dice, you should have even odds of only taking 1 damage or none. Step it up to 4 attack dice, and now you need at least 5, though 6 evade dice are better. Getting 4 evade dice on a non-cloak ship is difficult, but not impossible. Any more than that, and only the Fed's can really do it with crew and tech. Other factions have to cloak. 5 attack dice should have been the maximum in this game with the attack/evade dice the way they are. When you start stepping up to 6 or more attack dice, there really isn't anything you can do to evade it, so you have to use other methods (attack cancellation, discards to reduce the damage, and just bigger ships that can take the hit). The only faction in the game that can evade well enough without a cloak, use discards effectively, or reduce/absorb hits are the Federation. A close second is the Romulans with cloaks, Interphase Generators, and Reinforced or Enhanced Hull Plating. No other faction even comes close. Dominion has one big attack cancel, but there are enough disable/discard crew effects out there to make it iffy to use. The Klingons have the Projected Stasis Field, but it's a Discard, and it's almost a suicide mission for that ship. It becomes the #1 target of an opposing fleet too.
As a side note, with the USS Hood out, I think every Defiant will be running around with a Type 8 Phaser Array and Systems Upgrade. +1 attack die and +1 shield for 4 points? Sounds like a bargain!