Could someone provide me a chart of using the C-stick versus using A+direction, that compares their differences from a metagame perspective? Or are there no differences other than ones that are purely a matter of personal preference, and have no impact on the meta?
I can't provide a chart, but I'll give you all the scenarios in which c-stick is a better option than A+direction.
Glide Toss - The act of interrupting a roll with an item toss. The input for this is
1: Shield
2: Roll (press left/right while shielding)
3: Immediately after rolling (I believe you must still be holding shield), press A/C-stick to cancel the roll and throw the held item instead. You will no longer be invincible, but you will use the momentum from the roll to slide across the stage while throwing the item.
http://www.youtube.com/watch?v=zDtdmdkzaro
Notice how Diddy slides across the ground while throwing an item. If you want a pro Diddy/Peach/Rob/ZSS, you will see them glide toss often.
Diddy's glide toss is actually quite small. Characters such as MK, Falco, G&W and ZSS have glide tosses that are significantly larger. Others as well. I don't believe everyone can glide toss, but most characters can.
Why you would use C-Stick over A+direction
There are 8 different glide tosses. 2 directions in which to roll (left/right), 4 directions in which to throw the item (up, down, left, right). 2x4=8. Glide tossing with the C-stick allows you to roll left/right on the joystick while throwing the item in a different direction. Using A, a glide toss where the toss and the roll are NOT in the same direction is rather difficult. It IS POSSIBLE, but VERY difficult. Thus, with A, 2 glide tosses are made easy. With C-stick, all 8 are easy.
*Note: ZSS can't glide toss with her back roll, only with her forward, so she only has 4 glide tosses. There may be some other characters with this disadvantage as well.
Air Dodge Cancelling Item Toss(ADCIT) - Cancelling an airdodge with an item toss.
http://www.youtube.com/watch?v=h7tIIDzlab8
This video contains all the information you need on ADCITs. Same basic idea as glide tossing, c-stick makes life easier. Move in one direction while ADCIT in another.
Directional Influence (DI) - Changing the trajectory of your character when sent flying by an attack, which helps you to live to higher percents.Also helps you escape from multi-hit moves like Pikachu's down smash or Snake's neutral aerial, which helps you refrain from building up unnecessary damage.
http://www.youtube.com/watch?v=5hQIy7P7RWo&feature=related
http://www.youtube.com/watch?v=TMhy29SGkDU
For moves that send you flying, you aim for the corner, as the death zone at the corner is further away from anywhere else. Think about it like a right-angle triangle; the hypotenuse is always the longest side!
Momentum Cancelling - Preventing yourself from dieing at higher percents by regaining control of your character ASAP after having been sent flying. Done by doing your fastest aerial (to regain control), then using your best option to get back to the stage.
http://www.youtube.com/watch?v=njcYW0rFmUg
DI and Momentum Cancelling are very important at higher level play, as doing both optimally helps you live to MUCH higher percents.
Since they're so important, it is best to DI (from an attack that will send you flying) with the control stick, and momentum cancel with the C-stick. For example, Diddy Kong's back aerial is his fastest aerial. However, if he gets hit by a move that sends him sideways, and he is using back+A to momentum cancel, he will DI completely sideways (not toward the corner) which is horrible. Using C-stick, however, he can easily DI to the corner by holding up/diagonally up and to the side on the joystick, and use his back aerial with the c-stick.
DACUS - Dash Attack Cancel Up-Smash.
http://www.youtube.com/watch?v=WlGIl27bJQM Everything you need to know about DACUSing is here.
I'm not sure if it's possible with A, but I believe it isn't. There is also something called a BDACUS, which is just a buffered DACUS. If you don't know what buffering is, ask me.
There's some controversy over the name of this, apparently, so I'll just show you the video.
http://www.youtube.com/watch?v=TtxbcqWUQpE
ONLY FOR FALCO
How-to
1: Run
2: Dash attack while running
3: Let GO of your running direction
4: Up-smash immediately
Yes, it's possible with A. But c-stick makes it easier because you just have to flick it down, let go of the joystick, then flick the c-stick up. With A you have to let go of the running direction and press up on the joystick. Not a big difference, but meh.
DAL - Dash Attack Lock
ONLY FOR ZSS (Zero Suit Samus)
http://www.youtube.com/watch?v=pItF4Sa5NHY
If you try to do it with A, you'll do a side-smash.
Pivot Rolling - Turning around and doing a forward roll instead of doing a back roll.
http://www.youtube.com/watch?v=RmC8qOwtf8c
Honestly, I don't know if this is useful for anyone other than ZSS. I actually found this in a ZSS guide on these boards.
Basically, ZSS has a good forward roll and a horribly slow back roll, so instead of rolling backward, you PIVOT ROLL! :D
If another other character has a forward roll that is significantly better than their back roll, this could be useful.
Input
1: Back (to turn around)
2: Shield (to prepare for roll)
3: Forward (while holding shield, to initiate the roll)
So if I were facing the left, this would be right, shield, shield+right.
I would have to tap right twice quickly in order to do this quickly.
However, if you're shielding and you press left/right on the c-stick, you roll. Thus, I can do the first right with my joystick, and the second with my c-stick. Definitely easier.
And finally, using c-stick for aerial attacks allows you to use them more flexibly. Following the same logic as glide tossing, you can move in one direction and do an aerial in another. eg Move backward and use your forward aerial, fastfall and do an up aerial at the same time, fastfall and back aerial, etc.
If you're not using your aerial in a different direction than the one in which you're moving, then A is fine. If you're glide tossing in the same direction as you're rolling, then A is fine. If you want to use your neutral aerial (A without direction), you NEED A. People who play Mr. Game&Watch momentum cancel with A because his neutral aerial is his fastest. Smashing is fine with either c-stick or A+direction.
Conclusion
C-stick is better/mandatory for doing one thing in one direction and another thing in another direction. This includes glide tossing, ADCIT, DI+Momentum Cancelling, (B)DACUSing, that Falco thing, DAL, Pivot Rolling and aerials in a different direction than your movement.
A is mandatory for neutral air.
Usually, C-stick is the way to go.
Hope this helped! If you have any questions, just ask.