There's not a lot that separates him from Tentacruel, but his power is definitely something. Tentacruel can't really hurt common spinblockers with knock off but Armaldo can cause serious damage, and can also fill a far more offensive role than Tentacruel (who relies on either toxic or weak attacks). Armaldo also has more mixed defenses (though they're overall inferior to Tentacruel's) which is more fitting to the role of a pokemon that has to take both special and physical attacks.
There are small things as well, like priority, x1.5 SpD in sandstorm, doubled speed in rain and few exploitable weaknesses.
I won't lie though, Armaldo is outclassed. In OU I don't see him being very useful other than in dedicated rain teams maybe. However, Armaldo could probably find use in lower tiers. Sandslash offers serious competition though.
Realistically, Sandslash is almost certainly a better AV Spinner than Armaldo. He does have better special bulk than Sandslash and more Attack, but his typing is honestly awful (resists Normal and Poison, Neutral/Weak to everything else.) I don't think the Swift Swim niche is enough for him.
Though it's not like either of these Pokemon has realistic OU chances. Armaldo has too many flaws to ignore, while Sandslash is basically entirely outclassed by Donphan (who's a bit iffy anyway.)