Sand Veil isn't very good. Smogon's Evasion Clause bans it entirely in some formats. Even if you're playing a format where it isn't banned, it's still bad, as your team has no sand setters whatsoever. Sandstorm is also generally a waste of a slot, so you would have to get a Pokemon that automatically sets it up.
Rough Skin is generally better. Those little bits of extra damage can make all the difference, plus it can make physical attackers think twice about hitting Garchomp.
Hope I helped!