The offensive role is one of the best for Sun teams. Ice Shard is a valuable tool for them too, as you can revenge Salamence/Thundurus-T/Landorus-I/Garchomp and such as long as they are below 70%. Sturdy also means you will survive at least one attack from them, so you could 2HKO them with Ice Shard if needs be. Hippowdon is not a good idea, as you said you're using a sun team and you want Donphan on it. Hippowdon would be dismantled by most Fire attacks, and it also negates sunlight with sand stream. Offensive Donphan doesn't always need the bulk because of Sturdy, but Defensive Donphan loves the bulk.
Speaking of which, Defensive Donphan hates special attacks. Being weak to Grass/Water/Ice is bad enough, but Fire attacks will destroy it in sunlight with Stealth Rock up. The set you've chosen is a hazard setter and a rapid spinner, but the smogon analysis for offensive donphan recommends Stone Edge over SR. If you have nothing else to run SR on, leave the set as is.
Otherwise, both sets are good, but it depends on your team as Laprabi said. Donphan is a nice check to most Ice weak threats, but Mamoswine does what Offensive Donphan does, but better and with STAB. SR/Earthquake/Ice Shard/Superpower is a typical Mamo set, which can revenge healthier Thundurus and can OHKO Salamence/Garchomp (with some prior damage)/Landorus-I.
Generally, it depends on your team, but Mamoswine does the Offensive Donphan a bit better. Hope this helped a little.