Naganadel > Dragalge
First of all, Naganadel gets Tailwind, which already makes it an asset in DUU. Having this small bit of utility definitely has the potential to give you a big advantage.
They both have similar typing, and both are extremely dangerous with Draco Meteor and Sludge Wave, and but only Naganadel has access to Fire Blast/ Flamethrower, so it has an edge because it can deal with any Steel types that could otherwise give it trouble, whereas Dragalge has to rely on other neutral coverage.
Naganadel is faster and in a fast paced meta like Doubles, this could be to your advantage. The much slower Dragalge is probably better suited for a Trick Room team.
Although Specs Adaptability Dragalge hits like a freight train, Naganadel's Beast Boost makes it almost as powerful, and definitely more viable for spamming than Dragalge. Also, keep in mind that Specs or AV on either makes it difficult to slap on Protect and use either for scouting. Life Orb is probably a safer way to go about using them, but Naganadel more so than the slow Dragalge.
Dragalge is very susceptible to getting revenge killed. On the other hand Naganadel (barring priority) is much more difficult to deal with, specially if it racks 1-2 KOs.
There's a reason Naganadel is Tier 1 according to Smogon and Dragalge doesn't even make the list.
The only place where I see Dragalge shine is a Trick Room team, so definitely try that out too and see if you can catch any opponents off guard.
