A button is closer from the jump buttons so probably easier if your just learning DJC (at least for me it is like second nature now). Only times I use the C-stick for aerials if I want to do a dair and dont want the risk of fastfalling, e.g. edgeguarding and not die by the fastfall.
Like Bloshi said nair and uair are the most awesome to DJC. DJC'd uairs for juggling and nairs for alot of different stuff, like techchasing, punishing an approach by RDJCing it, shieldpressure (DJC'd nair -> jab - > dsmash/grab works particularly well imo). I dont use the dair much, but it's mostly used for edgeguarding and coming from very high up and surprising the opponent by suddenly fastfalling a dair. Full hop DJC'd bair when your not facing the opponent looks really weird/flashy. Its pretty hard to land, but also has its follow ups, like angled ftilt/fsmash. SH bair is the preferred one though. On characters like bowser, dk, ganon, roy, you have yourself a killer combo by just doing a SH bair to DJC'd uairs (to FH fair -> dsmash/techchase for 0-death xD). And like Bloshi said be very carefull with your DJC'd fairs. Ive been gimped by Marths plenty of times in the past by playing too agressive, as they can easily gimp you if they see this coming.