Donphan outclasses Sandslash in most ways. Statwise Donphan has 90/120/120/60/60/50 (HP, Atk, Def, SAtk, SDef, Spe) and Sandslash has 75/100/110//45/55/65 (same order). So the only stat Sandslash wins in is speed, which is pretty horrible anyway so that's useless.
Sandslash, though normally a RU Pokemon does however have a role in OU which is done by utilising Sand Rush which doubles speed in sandstorm. Of course you'd need to run it in a Sand team and it would function fine as an offensive rapid spinner with EdgeQuake + Swords Dance as well. Unfortunately, you're looking for Rapid Spin/Stealth Rock together apparently, so that rules Sandslash out except you could always run Stealth Rock over Swords Dance, but Sandslash's base 100 attack really doesn't cut it if you want to sweep.
Anyway, movepool wise Donphan gets access to the coveted Ice Shard - which lets it revengekill Dragon types and such. STAB Earthquake backs this up, and it runs Stealth Rock + Rapid Spin in the remaning slots. Unlike Sandslash, Donphan can also function in say, sun as it removes it's water weakness and allows it to get rid of hazards which are a pain for sun teams.
Sandslash's movepool is kinda barren :P.
Both still have no reliable way of healing so don't bring them onto super effective moves, or a strong special attack for that mater. They'll just die :c
So yeah, Donphan - it has more utility and will probably fit your needs better than Sandslash.