Gengar
Pros:
- Super effective on Fairy, one of if not the best types in Gen6.
- Levitate ability eliminates ground weakness, a powerful offensive type
- Access to Taunt, shutting down stall
- Hits steel for neutral damage thanks to the steel type nerf
- Good spinblocker
- Unpredictable thanks to many viable sets, and can even run Destiny Bond
- Extremely powerful mega evolution you might want to consider using (learn more about that here)
Cons:
- Exceptionally frail
- Walled by Assault Vest Users
- Shut down by priority users
Note: Espeon isn't necessarily less predictable than Gengar. Gengar has many different sets, so if anything, Gengar is rather unpredictable.
Sample Movesets:
Your movesets could use editing, so here are some Smogon recommended sets.
Gengar @ Life Orb
Ability: Levitate
EVs: 252 SpA, 252 Spe, 4 SpD
IVs: 29 HP, 0 Atk
-Shadow Ball
-Sludge Wave
-Focus Blast
-Taunt
However, Mega Gengar is just a better version of Gengar, so I reccomend using this set:
Gengar @ Gengarite
Ability: Levitate >>> Shadow Tag
EVs: 252 SpA, 252 Spe, 4 SpD
-Destiny Bond
-Shadow Ball
-Sludge Bomb
-Taunt
Espeon
Pros:
- A great ability in Magic Bounce
- Psychic typing is super effective on fighting, which is somewhat popular
- Can take a few Special Attacks after Calm Mind
- Access to Morning Sun for recovery
- Access to Trick allows it to choice lock opposing Pokemon
Cons:
- Shallow Offensive Movepool; has trouble against Steel
- Weak to the common dark and ghost types
- Lackluster defense hinders its ability to switch in against some Pokemon
Sample Movesets:
Espeon @ Leftovers
Ability: Magic Bounce
EVs: 164 HP, 92 Def, 252 Spe
IVs: 0 Atk
-Calm Mind
-Psychic / Stored Power
-Dazzling Gleam
-Morning Sun
Espeon @ Choice Specs
Ability: Magic Bounce
EVs: 252 SpA, 252 Spe, 4 SpD
IVs: 0 Atk
-Psychic
-Shadow Ball
-Dazzling Gleam
-Trick
Gengar is better than Espeon. It is in a higher Smogon tier, has a much better typing, and a powerful Mega Evolution. However, Espeon does have a niche in BSS as a Magic Bouncer and Calm Mind sweeper. Both are good Pokemon to use.
For more BSS set alternatives, go here and here.