Thinking of buying the game, should I download it or buy it at a shop?

I think downloading it would be better because it means that the game will always be on the 3DS but if the 3DS brakes or if I have to get a new one for some reason, does that mean I can't play downloaded games such as NSMB2 on a new 3DS?
I'm pretty much asking if I download NSMB2 on my 3DS, can ONLY that 3DS play it?

Could someone tell me how downloaded games on an SD card work?

I personally got it physically. I dont like retail games downloaded since if you lose the 3DS, you lose the game too. And i like seeing the boxart n stuff.

I think you just buy the game on the eshop and it gets downloaded to the SD card.
_________________________

Wouldn't make sense if Nintendo tied downloads to a 3DS since alot of people upgraded to the 3DSXL.

I have no idea how it works, hopefully someone here knows how it all works.

You can do a system transfer to transfer all your downloaded stuff onto a new 3DS...
_________________________

Oh right completely forgot about that. Still don't know which one to buy though, might open a vote thread and see which type of purchase has more cons than pros.