I've been considering getting Super Smash Bros. for 3DS, but using the Slide Pad for movement has proven extremely inconvenient in the demo.
Unfortunately, the demo did not allow for configuration and I couldn't find any definite answers online, nor did the salespeople know whom I asked.
In case I can't use the D-Pad, I've decided to wait for the Wii U version with which I'll just play using a simple Wiimote.