I'd say cartridge, as you can get them cheaper and there's less risk of data corruption. But if you're the type who loses things easily, maybe digital would suit you better![]()
I personally prefer to have physical copies of most of my games. I like the cases they come in (they look pretty on my bookshelf!) and it gives me the option to resell the game at a later point.
If you're the kind of person to lose things easily, though.. Then I'd definitely go digital. A 3DS should be easier to find than a small cartridge.