I'm guessing you come from a more PvP or at least shooter background. I do like the feel of the Mattock more than the Harrier myself. Usually in proper shooter games, using a semi-auto rifle would reward you with higher DPS than a full auto rifle like the Harrier because it takes more skill to master, but not ME3 though... 
Stiil, I've always felt there is something more rewarding about the Mattock than the Harrier, probably from my tendancy to like semi autos more than full autos in past games.
Yea, I've been a shooter game player for a long time. I'm much more a fan of precise shooting than spray and pray. One of my favorite full-auto weapons is the Phaeston, and that's because it, along with the Avenger, work more like a traditional full-auto assault rifle than most any other rifle in the game. And as such, you can burst fire it if you want to. You can't really make it do semi-auto unfortunately, but you can use short, controlled bursts, which I often do, especially on infantry level enemies.
That's one of the reasons I can't really click with the Typhoon. It has some great benefits, but that stupid ramp-up mechanic (which makes exactly zero logical sense) prohibits burst firing.
And something people may or may not realize is that in real life, soldiers in combat don't go full-auto most of the time. Full-auto is mostly for suppressing fire rather than putting on a target. More often than not they have their rifles set to semi-auto. It conserves ammo and discourages them from just blasting away. It's also much better for accuracy.
So for those reasons, I prefer the Mattock. That and I love the way it sounds. In the right hands, like the Human Soldier, it's still gold viable.