I can give a general outline of each Pokemon, but keep in mind that when building a team you want to choose Pokemon that are in synergy and cover each other's weaknesses. Ideally, I would look at the rest of your team and decide who fits in better. I will list their strengths, weaknesses, moveset and ideal team partners.
Strengths: Houndour's main niche is dishing out high amounts of damage immediately to break through walls. It's dual typing gives it great coverage, and it has access to Sucker Punch, meaning it doesn't have to heavily invest in Speed. It's high Special Attack stat gives it lots of power.
Weaknesses: Houndour has terrible bulk, made even worse by its bad defensive typing, meaning it has to come in on a free switch. It's also weak to Stealth Rock.
Moveset: Houndour @ Life Orb
Ability: Early Bird
EVs: 36 Atk / 196 SpA / 252 Spe
- Fire Blast
- Dark Pulse
- Sucker Punch
- Destiny Bond
Teammates: A user of Rapid Spin or Defog is ideal so Houndour doesn't have to worry about Stealth Rock. Users of U-Turn or Volt Switch are excellent so Houndour can come I'm safely.
Strengths: Cyndaquil is a great wallbreaker with STAB Eruption. It also has a good defensive typing in Fire.
Weaknesses: It's poor bulk not only negates its good defensive typing but makes it hard to use Eruption, as Cyndaquil is not usually at high health. It's weakness to Stealth Rock is also a huge detriment, making Eruptiom completely useless. It also doesn't have much in terms of coverage.
Moveset: Cyndaquil @ Choice Scarf
Ability: Flash Fire
EVs: 44 HP / 32 Def / 196 SpA / 236 Spe
IVs: 1 Atk / 30 SpA / 30 SpD
- Hidden Power [Ground]
- Fire Blast
Teammates: An entry hazard remover is almost necessary; otherwise Cyndaquil's Eruption will be very weak. Users of Volt Switch and U-Turn are good so Cyndaquil can come in safely.
Overall, Houndour is the better choice because of its higher Special Attack, access to Sucker Punch and better offensive coverage. If your team has a weakness to Fighting, Cyndaquil might be the better choice.