Steel wing is actually better than dragon tail on offense?

No, SW is the best defensive moveset, mostly in the area where Shellders, Seels and Jynxes are uncommon, but without STAB. DT is another decent defensive moveset while with STAB, which gives an additional multiplier.

It all depends on what you value.

Taking the least damage on your pokemon
OR
Winning the fastest.

Dragontail is more energy and more dps meaning it's wholly superior to Steel wing in every way.

However, steel wing is faster and easier to use while dodging everything.

On defense, DT is even more energy and more dps than Steel Wing and even larger an advantage due to the way the Gym AI works. And when you factor in a lot of Dragonite counters have a water typing to resist steel the edge becomes larger.

Agree with the dodging part, but steel moves are pathetic on offense. It is unusable against fire, water, electric and steel (though not many defenders electric/steel). For cases it will be SE against (Ice, Rock), you wouldn't use a dragonite. The worst part is probably you can't take down another dragonite as quickly with steel moves!

Dragon moves are usable against everything except steel (not many defenders exist) and fairy (again no defenders exist), and you wouldn't use a dragon against a fairy in the first place.

I for one am glad to add a SW/Rage to my dragonite lineup. There's a ton of rhydons everywhere... now I don't necessarily have to switch out and use a water mon.