I'd suggest ModChipsDirect for both cards. (Sky3ds $84.99, Gateway $64.99) They don't have the cheapest price but they have fast shipping! I got my sky in 3 days after ordering which was worth the slightly higher price instead of paying less but waiting for weeks.
What type of 3ds do you have? That would change which card I would have gotten. I have the N3DS so I got sky but I would have gone for Gateway if I had any other 3DS/2DS.
I'm trying to buy something from there but the whole international transaction thing is making it a damn nightmare. My bank is allowing international transactions until 4:30 PM EST but they take ages to respond for whatever reason.