Guess it depends what you want them for? Quagsire would make a good HM Slave:
Sandlash can only learn:
As for battle, it depends on the rest of your team. Generally though, Quagsire.
Quagsire @ Leftovers (or whatever you can get really)
Ability: Water Absorb
Quagsire has a x4 Grass weakness, but Ice is now x1 and Water x0 with its ability. If you have a flying or fire type on your team, Quagsire would do good taking some ice and water hits.
While Sandslash has slightly higher stats, its ability is near useless, and it has low HP and Sp. Def, coupled with mediocre speed. At least Quagsire's slow and you know it, Sandslash is surprisingly slow. :/