They have some varying roles. Lets start with Gengar.
Lets show off some of his stats.
HP : 60
Atk : 65
Def : 60
SAtk : 130
SDef : 75
Spd : 110
So we see a very good base stat in speed and special attack. This gives us the powerful role of Special Sweeper. No set up required. However Gengar can also run a highly effective SubDisable set.
Now lets have a look at Chandelure, then we'll compare them.
Chandelure Base Stats
Atk : 55
Def : 90
SAtk : 145
SDef : 90
Spd : 80
So we see some higher defensive stats. We also see a higher special attack but a lower speed. This means that Chandelure will be outsped by many of her threats. We can fix this problem by either giving it a Choice Scarf, or EV training in speed. For in-game usage I would suggest option B. As no in-game Pokemon is EV trained, you can remedy many problems simply by EV training.
So you have several bulky Pokemon on your team. You also have some fast sweepers, Jolteon and Gyarados.
However, other than Gyarados, and through simple bulk Deoxys-D, you don't really have anyone capable of taking a Special Fire attack. At the same time, no one on your team is capable of taking a STAB ghost move. Other than dear Scrafty.
So my suggestion, is Gengar. You're weaker to ghost moves than you are to fire moves, so Gengar with his high speed would be able to KO a ghost type before they KO him. He also works as clean up. With speed and high special attack, he can come in and without haveing to set up, he simply kills the opponent.
So, Gengar works very well on your team, and I hope you do well. But so does Chandelure as he provides a fire coverage with his ability Flash Fire.
I lean more towards Gengar, but Chandelure is also very functional.