Naturally, any UT game is more defensive than offensive. That's what happens when backing away and moving forward happen at the same speed. While you're moving towards a guy, his shots come at you twice as fast, while your shots come at him twice as slowly. UT will never be truly aggressive like Quake because of the movement.
But, as you just mentioned, between flak alt-fire, hitscan, and shield gun, UT2004 doesn't afford much to aggressive players. The slowness of the projectiles only screws with this even further. However, that stuff has changed quite a bit in UT3. Rockets and flak are fast and do insane damage, and it seems like the flak alt-fire is designed with aggression in mind given that it shoots a lot further in comparison to UT2004. Even the shock core's new speed promotes more aggression because with a faster speed it's easier to overshoot combos on incoming enemies.
I've played a fair few UT3 1v1s but haven't found them fun. I was quite surprised to see how slowly they played out, because with the absence of the shield gun I was expecting this insanely fast and fun spawn rape festival of awesomeness. To see that UT3 1v1 was not awesome left me very disappointed. I've always tended to play more aggressively in UT2004 than I should have by moving forward and shooting while everyone else seemed fine with throwing up their shield gun and backing around a corner, so not a whole lot changed for me. Except for the fact that I now have weapons that feel inconsistent and a movement system that leaves me with no options. Kewl!
I don't think you can argue that UT3 isn't more aggressive than UT2004. As I said, every UT game is defensive in its own right and they always will be. If you negate that stuff since it's always true, UT3's aggression factor far outweighs that of UT2004.
If you were calling me a low/low-mid player, I consider those fighting words. We might have to throwdown in UT2004. I predict 20-0 with me victorious! I hardly even know UT3's maps, so that wouldn't work too well.