CO2 systems are preferred by some vendors because they can never drive the PH so low that there is a risk to the pool. An acid feed system that gets stuck on can run the PH down to dangerous levels. This is a rare failure mode, they usually break so that acid is no longer being feed. And even if there is a problem you will probably notice it before the PH becomes dangerously low. Still, CO2 systems never have this problem. Continued CO2 injection will simply cause more CO2 outgassing and the PH will stabilize at a safe level regardless of how much CO2 is fed. There is also some minor personal risk when filling an acid tank. The risk of a CO2 tank exploding is far far lower, to the point where you can essentially ignore it.
Personally, I prefer acid feed. Acid is readily available and inexpensive. CO2 tanks are much less common and end up costing more. CO2 systems are also prone to improper installs, where much of the CO2 bubbles right out instead of dissolving in the water. And finally, acid lowers the TA, which tends to reduce the rate at which the PH rises. In a typical install the PH will get more and more stable over time. CO2 on the other hand does not affect the TA. If you combine a CO2 system with high TA fill water, your TA level will go up constantly and thus the amount of CO2 used will continue to increase until the system is overwhelmed and the PH goes up despite the CO2 feed system.
Don't misinterpret what I wrote. All of these problems are relatively minor/rare. Most people will be happy with either kind of system (assuming you can afford it).