Litwick comes later in the game than Solosis (Celestial Tower and Route 16 respectively), but it's generally better in the long run. Solosis starts with one of the highest Special Attack stats of any NFE Pokemon, but it can't use it very well due to a shallow movepool and low Speed. In Unova, where the best way to grind is Audino, Solosis simply cannot do much at the start -- unless you get a lucky Hidden Power type, it's too weak to take down Audino without switch training.
Litwick, on the other hand, is a Ghost type with Will-O-Wisp from the start. This allows it to slowly but surely whittle down Audino's HP with no risk of being knocked out, meaning it can level up much faster (Professor Juniper gives you the Lucky Egg in the same place as you catch Litwick, so it's even easier).
Solosis evolves sooner than Litwick does. In fact, by the time Litwick reaches Level 41 to evolve into Lampent, a Solosis at the same level will have had enough EXP to evolve into Reuniclus. Duosion is a powerful asset before you get Lampent, but it doesn't matter too much because it's easy to go to the Strange House and take a Dusk Stone to evolve Lampent into Chandelure.
Reuniclus' level up movepool isn't the best, but it does naturally learn Psychic at a fairly low level. Conversely, Chandelure doesn't learn any moves through level up, so you'll have to get what you want with TMs and Lampent's movepool first. With the right array of TMs, either can work, though Chandelure is typically more effective at taking out enemies.
In summary, they're both strong assets for an Unovan team. Each is a powerful special attacker obtained at around the same time, with varied movepools they can use to their advantage. Overall, as I've played Unova extensively and used both Pokemon in the past, I think Chandelure very slightly edges out Reuniclus. This is mostly because it's faster, allowing it to OHKO an opponent before it has to take damage.
However! Why can you not just use them both? They don't overlap types with each other, and both are very strong. If you must pick one, Chandelure is a bit better, but both on the same team form a very strong special core that's hard for computer opponents to break through. With that in mind, I'll provide an endgame set for each:
Chandelure @ Spell Tag
Ability: Flame Body
- Flamethrower / Fire Blast
- Shadow Ball
- Energy Ball
- Will-O-Wisp / Psychic
Reuniclus @ TwistedSpoon / Smoke Ball
Ability: Magic Guard
- Psychic
- Shadow Ball
- Signal Beam
- Recover / Energy Ball
