Personally I always classified those as a deadlift variation. As far as I'm concerned, if you're holding the weight in your hands and down below your waist, it's a deadlift. If the weight is supported on your torso (even Zerchers fall into this category) or above your head (overheads), then it's a squat. But, hey maybe that's just me. I just find it easier to think in those terms.
Like someone said, dumbell squats are pretty much identical to trap bar deads, the only real difference is the equipment. It just doesn't make sense to me to categorize them as different exercises.
As far as the pros/cons, I'd say that the amount of weight that you can use on the dumbbell exercise may be limited by your hand strength (similar to barbell or trap bar deads), this may be beneficial for building grip strength. But, more than likely it'll also result in suboptimal loading of your lower body. With a barbell squat (back or front) the weight is supported by your physical structure, thus allowing for much greater loads to be utilized.
The dumbbell exercise does allow one to keep a very upright torso position however, so if flexibility is an issue, or one just can't get the hang of front squats, this exercise may be a good alternative.
I also agree about what was said about the dumbbell exercise being a good break from the heavy spinal loading associated with back and front squats. But, honestly if I want an exercise that really works the quads and places minimal stress on the spine I personally prefer pistols or weighted pistols.
Obviously the disadvantage with those is balance, so I'm not saying that there is any one "best" exercise either. Just preferences.