Let's compare them step by step, shall we?
Rotom - Heat
HP 50
Attack 65
Special Attack 107
Defense 105
Special Defence 107
Speed 86
Total: 520
Typing: 2 weaknesses, 7 resistances, and 1 immunity
Pros:
- Good Speed and Special Attack
- Decent movepool
- Special Attacker
Cons:
- Overheat has low PP
- Overheat doesn't gain STAB
- Using Overheat makes other moves' power go down
Flareon
HP 65
Attack 130
Special Attack 60
Defense 95
Special Defence 110
Speed 86
Total: 525
Typing: 3 weakness, 4 resistances, and 1 immunity (Flash Fire)
Pros:
- Excellent Attack
- Decent Special Attack
- Decent movepool
- Mixed Attacker (mostly Special, which is a waste of its Attack)
Cons:
- Low Speed
- Poor Physical movepool, wasting it's abundant attack
- Good attacks learned very at very late levels
This is where it gets confusing:
- Flareon does not have any strong Physical STAB moves worth noting, with the exception of Fire Fang
- Rotom - Heat gains access only to Overheat, which is not a STAB.
Any way, I'll leave the choice to you. Personal suggestion is, though, get another Pokemon as the primary Fire Type. Both of these are not good enough to be the Primary Fire type. If you still think that you must use either of these, go with Flareon, as PP is very important ingame, and PP Ups, Ethers, etc. are quite rare.