There are advantages and disadvantages to both. Modern semiautos are more reliable than ever, but they can malfunction. When they do, most of the time it's pretty quick to clear the malfunction. They are usually faster to reload, assuming you are carrying a spare loaded magazine. That isn't to say that an experienced revolver shooter can't reload pretty quickly with a speed strip or speed loader.
Revolvers aren't usually ammo sensitive, but some can be. A revolver is almost always reliable, but it can malfunction, and a lot of the time it can't be easily cleared in the field. Some revolvers have a 7 or 8 round capacity, equaling that of the Shield, although those revolvers tend to be larger.
Personally I prefer to carry a Shield and a backup magazine. As others have said, it really comes down to what works best for you, and what you like. Also as mentioned, most of the SD encounters expend an average of 2 or 3 rounds...long drawn out gun fights are very rare for civilians. So either one should suffice for almost everyone, other than LEOs.
