You choose whichever one you want. If you don't like the feel of gun X, then chances are that you're gonna shoot like sh¡t with it.
Personally, I chose the Matrix. It is faster and no one can go against that fact. If anyone says "well, my friend could shoot an Impy just as fast as a Matrix" you need to realize that is PERSONAL ability. The MROF of a Matrix is 22 bps. Impys are lower.
With a Matrix, you don't have to rely on electronics to stop chopping. ACEs are bull crap to me. they completely screw up your rhythm and are too sensitive. You get a drop of any liquid or a spec of dust on the eye and it could misread the paintball. The low pressure operation of the Matrix is flawless. It doesn't matter what happens inside the gun, it will not chop.
As for efficiency, with a STOCK bolt kit, I actually get about my HALO with two 140rd pods (that's 180+280=460) with over 1000 psi left over (this is a 68/3000 max flo). With a bolt kit you get 900 shots per 68/3000 fill. It's basically the same efficiency as an Angel.
most of my team uses imps. On sunday we practiced and my teammate and I could hit this twig at the end of the field in 2 shots or less (not exaggerating). He has a Freak Factory Imp (used to have a matrix) and I have a Rat Imp.
Once again, that is YOUR personal ability to aim, not the guns. As long as the weather is permitting and you have high quality paint and a high wuality barrel that fit tightly, the accuracy will be the same in any gun. I'm able to hit a small twig about 150 feet away easily on my first or second shot, but once again, that is personal ability.
Of course everyone who says the Matrix is good will own them. that's because they shot them. Having someone that has nto shot an Impulse say good things about an Impulse is worthless. He/She doesn't know about it, therfore they don't have a balid opinion. Everyone that has shot my gun has wanted one. I've been offered many guns just for mine, but I keep it because I love it.
As for maintanence, it's not high. It just requires love. The best part about hte bolt assembly is that if you have a leak, you know exactly where it is. It will take you no more than 2 minutes to fix any leak in the entire bolt assembly. With an Impy there are many different areas where problems could occur and sometimes they are not fixable until you send them in. Now, the older Matrices had that problem too, but they have been getting very reliable lately. 98% of the problems on them are either the users fault or a simple leak through an o-ring.
Don't get me wrong though, Impys are high quality guns and I love them. But I swore to myself that I would not buy anything from Smart Parts (except for a Max Flo) due to their lying and capability to false advertise. I don't like SP's quality, but I have been impressed with the Impys that I have seen. My friend owns an old Shocker Turbo and it's a pretty nice gun also. I've shot Impys and seriously considered buying one. But once I shot my Matrix again, it made me realise why I bought a Matrix in the first place.
You can't say gun x is "better" than gun y. It's an opinion. there is no wrong, but there is also no right. You choose which gun you want to choose. I'm not going to say my Matrix is "better" than your Impy. I love the way my Matrix operates, I love the feel my Matrix has, and I just plain ol' love my Matrix. Now, instead of arguing over which gun is better, go out and just play. It's the person behind the gun that makes the gun