I would say that Glimmora is the better Pokemon to choose. Despite Nacli being found early in the game, it is painfully slow and fits into the slow, bulky type of mon. Meanwhile Glimmet can be found in East Province Area 3, which is around early mid game. It also evolves pretty soon after you get it, at level 35 and has great special attack, great coverage, and decent bulk and speed. Garganacl can be useful, but for in game teams, offense is usually way more useful than defense, and Glimmora fits that role. Glimmora is also super effective against Grusha, resists Larry's normal moves (don't use against Dudunsparce because Drill Run will most likely OHKO), hits a lot of Geeta and Nemona's team, hits all of AI Turo's Pokemon but Iron Bundle for SE, hits most of AI Sada's Pokemon pretty hard, and is just good in general. Here is a moveset for both of the Pokemon.
Glimmora @ Filler
Sludge Wave
Power Gem
Earth Power
Dazzling Gleam / Filler
Garganacl @ Leftovers / Filler
Body Press
Earthquake / Recover
Stone Edge / Rock Slide / Salt Cure
Recover / Filler
