There are certainly situations where it feels like the a button can be better than a c-stick, and vice versa.
Honestly, it's personal preference. Obviously when you're jabbing, it's usually much better to use the a button. If you set the stick on attack and want to nair, however, some would say that nairing with the a button is better while others would say that using the c-stick is simpler.
It's all about preference and what kind of character you use. I use Palutena and her tilts aren't used as much, so I just set my c-stick to smash attack. That means I always use her tilts with the a button. Also, even when I smash attack, I tend to mostly use the c-stick for it. There are a few moments when I habitually use the a button for it, however.
Just try to experiment with multiple control schemes and use something that fits you the most. I've tried setting c-stick for attack, but it didn't fit me at all. As you get better, you won't really think of this issue that much.
