I am struggling to decide between the two players. I know it's mainly down to personal preference but which player is considered the better option?

I've read both take a 128gb micro sd card, is that correct? Also once an SD card has been formatted on another mp3 player, can it be used straight away in a new one or do I have to format it again?

Which player handles playlists better?

Thanks for any advice.

If you have easy to drive headphones/earphones, then they will sound less or more the same with both players. More demanding phones you plug, better dx50 should sound over A15. If i would need a player with huge power output, i would go for Fiio X5.
 
I think, that my es10 would benefit from more amping, but players like Fiio or Ibasso are overkill for them, so i will rather have slightly worse sound, but much better portability and battery endurance with A15.