If you asked me, I thought the Enterprise seemed better able to handle the damage than the Reliant. I mean she absorbed a brutal phaser attack, and a photon torpedo, and though she was crippled by the attack she returned fire with only a few hasty shots and was able to force Reliant to withdraw.
In the Nebula, teh two ships exchange fire when they nearly collide. The already badly damaged Enterprise seems to fair a little better in that skirmish. The shots she fires at the Reliant seems to really seal its fate by that point, and the final attack when the Enterprise comes up behind Reliant is just the knock out blow.
Looking at the model though, it really does look like the Reliant is designed with more of a tactical/strategic role in mind, with her dual shuttle bays and compact hull form. She seems to have as many phasers on her saucer, with a dedicated weapons platform on her roll bar, there may even be additional weapons on the raised part of the hull.
It could be suggested that the Miranda Class is more of a "workhorse" for the fleet than the Constitution Class whose primary duty is exploration.
If we were to contrast the two vessels, the Constitution would tend to travel out into deep space to explore and provide support to far flung federation colonies, whereas the Miranda would be designed to stay close to Federation territory to provide help and support for core Federation worlds. The Constitution Class with it's increased volume would have more room for dedicated to the sciences, thus is better suited for exploration where the Miranda's smaller volume would mean she is limited in her scientific duties and is only suited to relatively menial tasks such as finding a suitable planet to test the genesis device. In wartime, it would actually fall to the Mirandas to provide most of the military support, wheras the Constitution Class will fill a role analageous to Germany's surface raiders in the far seas during the first world war.
This is of course all speculation.