Even if Defiant did have the capability of firing at other angles (IE phasers or torpedoes), it still doesn't change the fact that E-E is everything Defiant is, just more of it. Bigger hull able to handle more damage, more power output, more phaser arrays and torpedo bays, large crew to handle battle damage, more of just basically everything, less cloak, which isn't necessarily an advantage, if E-E is able to track it, anyway.
That doesn't mean Defiant won't get its licks in, and it would be an easy win for E-E, just that the odds are with the Enterprise-E.
Now, if we were talking Enterprise-D, it just might be a different story and go in Defiant's favor (Caveat: Except maybe "All Good Things..." Enterprise D, which also has a cloak and has been upgraded to basically a dreadnaught).
That said, I think the real contest might be Voyager VS Defiant. That might be closer, but my money would be on Defiant...not because I like Voyager less (I think Voyager is a better looking ship, personally), but because Voyager is more a multi-role explorer ship, whereas Defiant is all warrior.