Both are great Pokemon, but I'd give it to Dragalge.

- Which has the better design?

I like Kingdra's cleaner and more elegant design, but am also a fan of Dragalge's weedy and grungy style. It also looks more like a dragon in my opinion, which is a plus.

- Which one is easier to train?

Hm... Well, I haven't trained either, but I'm going to call it a tie. Dragalge has more weaknesses, but also more resistances and a larger move-pool to counter different types of Pokemon. Kingdra only has two weaknesses, but a rather small move-pool in terms of type diversity and can't hit fairy-types with anything.

- Which one has the better type?

As poison is my favorite type, I would have to go with Dragalge.

- Which is more useful throughout the game?

Not too sure, as it would depend on the user, their team, and the region.

- Which is more useful in the competitive environment?

Like above, I think both are equally as good. Dragalge can hit hard with Adaptability, but Kingdra can hit fast with Swift Swim. Not too familiar with the competitive scene, but Dragalge is ranked higher by Smogon if that means anything.