With the standard front squat, the rhomboids are the weak link. Once the rhomboids become fatigued, or if they're too weak to hold a heavy barbell in the rack position, the bar will come forward (the rack position is lost) and the set is basically over.

Sure, the barbell front squat is one of the best movements in regards to athletic application. It has a great transition from weight-room strength to "on the field" ability. But if we're talking about focusing on just hitting the quads for size gains, then having that weak link makes it inferior to the Smith front squat. And that weak link is eliminated with the Smith machine.

It's far easier to hold the bar in the rack position with a Smith machine than with the barbell. Multi-time strongman winner Jouko Ahola uses the Smith machine for his front squats during training. And here's IFBB pro Mark Dugdale cranking out some reverse-band front squats on the Smith:

The other advantage here is that you can do high reps. Doing a set of 20 with the barbell front squat means the upper back is going to tire out long before the quads. With the Smith, this isn't an issue. You can get the bar squeezed in very close, raise the elbows high enough to hold it there, and focus on what the quads are doing instead of focusing on holding the bar.