You should use the .nds one.

 

If it was a slot-2 Supercard, I'd say either one would work fine, though if you had an issue with the *.nds you should try the *.sc.nds one instead. However, since you have an SCDS, use the *.nds one.

*.sc.nds is the same thing as *.ds.gba which is meant to run from a slot-2 device with GBA compatibility. The only reason *.sc.nds exists is because slot-2 Supercards used to require *.ds.gba ones but would only launch them in DS mode if they ended in *.nds, so the *.sc.nds ending was born. It's merely a rename, and it's mostly unnecessary now since the slot-2 Supercards now launch both of them fine as of firmware 1.81 or so.

 

Thanks for reviving an old topic. But as already mentioned, .sc.nds is only for old slot-2 supercard flashcards (like supercard SD, supercard lite). Today, the only file you need is .nds.