Both these pseudo-legendary Pokemon are great individual, so I'll break it down for my opinion, and for you to choose.
Kommo-o
- better in both defenses
- good resistances
- good STAB moves and move pool
- satisfactory abilities
- quite a bit weaknesses
Hydreigon
- has higher of it's appropriate attack (Sp. Atk)
- slighty faster
- good resistances/immunities
- good STAB moves and move pool
- quite a bit weaknesses
Kommo-o is a great Pokemon if you want a good attack who can withstand attacks. It's abilities Bulletproof/Soundproof are based off of the attacker, so knowing when it's useful is unpredictable. However, if you do have the HA Overcoat, consider using Kommo-o in double battles with a Pokemon who can create a damaging weather condition (Hail/Sandstorm); otherwise, Kommo-o can not learn any such move to do this alone. However, there are no fast Pokemon for your ally who can enter with such a weather condition, and protect Kommo-o from Fairy type moves, as it can be very treacherous. So the obvious initial choice would be to teach Kommo-o Poison Jab, as it's the only appropriate choice. For the other moves, you most likely would chose Dragon Claw and Sky Uppercut, and finish it's move slot with something like Rock Tomb to take care of it's Ice and Flying weaknesses.
Hydreigon is also a good old powerful Pokemon, and it's ability Levitate does change it's neutral resistance from ground to an immunity. Moves like Dragon Pulse and Dark Pulse are both popular strong STAB moves. It's other moves can be replaced with good moves such as Flash Cannon to deal with the ice and fairy weaknesses, and maybe Flamethrower to take out an extra weakness, Bug. With it's speed and attack, it's more appropriate for single battles, but can be used well in double battles to help defend it's weaknesses as well.
Overall, both Pokemon can be used for both single and double battles. For Kommo-o, I would use in single battles if it has the ability Bulletproof, since all moves it protects are single target moves - excluding Victini's V-Create. If it has Soundproof/Overcoat, it's more appropriate to use in double battles since more popular moves that soundproof protect are multi-target moves such as Hyper Voice, Snarl, Boomburst, and Perish Song. Also, you'd more likely see a Pokemon with Snow Warning/Sand Stream in double battles. Overall, I think Kommo-o is more suitable for 2v2. Hydreigon can a good combination of speed, attack, and moderate defenses, so it can be great in singles; However, use it in doubles if you feel it's more vulnerable. But, in my opinion, Hydreigon is your 1v1 Pokemon.
Like I said, both are great in both scenarios, and it's ultimately your choice - so there's no bad decision in what battle style you choose for them.
Hope I helped! :)
