I'd go with a QQ3ds. I just got one and the DS and 3DS carts work just fine and as expected. I'm running on a N3DS with 10.1 Just watch out as all the QQ3DS sellers are shady and you'll end up paying an extra $10 for mystery fees. Still a great deal though.
Sadly if you read my thread about QQ3DS purchase experience you will see that the sellers of QQ3DS kinda suck... BUT, you'd end up getting it and it will work.
To clear up any confusion the QQ3DS sold on the open market includes a microSD reader, a QQ3DS card and a QQDS card. The QQDS cart obviously is for playing DS games. It's a clone of an older TTDSi some similar card, but it works just fine. the QQ3DS cart itself is identical to a sky3DS.
Be mindful of the distinction between QQ3DS and QQDS in my sentences above!!!!
You need to buy a Sky3ds and a R4 3DS card,sky3ds to play 3ds games,r4 3ds card to play DS games.
No flashcard can play both DS and 3ds games on 3DS v9.9.0.