Well, as Imperial Fleet Admiral it is up to us to determine how best to outfit our fleets with an effective fighter screen. So lets begin with an examination of these two fighters stat line.
Tie/Ln - The Line fighter of the Imperial Fleet, swarming the enemies of the empire into oblivion.
8 Points
3 Hull
3 Attack
4 Speed
Swarm
TIE/In - Sienar system took the advances made with the TIE/x1 were integrated into the superiority interceptor.
11 Points
3 Hull
4 Attack
5 Speed
Swarm
Counter 2
Now, lets just look at each stat one-by-one.
TIE/Ln vs TIE/In
Points : 8 v 11 - 37.5% More
Hull: 3 v 3 - 0% Improvement
Attack: 3 v 4 - 33% More
Speed: 4 v 5 - 25% More
Both have Swarm, and it is hard to put a numerical value on Counter.
Attack Damage with Swarm: 1.9375 v 2.46875 - 27.5% More (Swarm has a larger effect with a smaller dice pool) - Calculation I used 50% * # Dice + 0.5 * Probability of at least 1 Miss (1 reroll results in .5 extra damage on average)
This means based off those stats alone the TIE Fighter beats the Interceptor point-for-point. But Counter may tip the balance, the ability to retaliate against any attack, even one that kills it, with two dice make Interceptors quite useful.
Now, how do they stack up against each other in combat? Assume every round they are all in range of each other, a strange and unlikely idea, but it simplifies the math, and makes it doable.
12 TIE/Ln (96 pts) vs 9 TIE/In (99 pts)
36 Hull vs 27 Hull
12 Fighters with 3 dice each + Swarm results in 23.25 Damage.
9 Interceptors with Swarm results in 22.21875 Damage.
12 Counter Attacks (1 for each Fighter attacking them) at 2 Dice each, plus Swarm, results in 16.5 Hits (1.375 Hits each)
We have the Interceptors suffering 23.25 Damage, while the Fighters are taking 38.71875 Damage.
1 TIE/In Survived this epic battle, while 0 TIE/Ln did.
This shows if Imps were going up against Imps and they both decided to max their fighter support and go purely with superiority fighters the Interceptors would win (on average).
Since that scenario isn't real likely lets set a limit of 60 points (because if you are running big ships you won't want to use too much on Squads, and if you are running Bombers you want to leave room for them), and we'll look at how it works against various Rebel line ups. Assuming both squads attack at the same time, not terribly accurate, but calculating for Initiative is iffy at best.
Imperial Set Ups
7 TIE Fighters - 56 pts, 21 Hull, Constant 13.5 Damage per Turn
5 TIE Interceptors - 55 pts, 15 Hull, 12.34 Damage per Turn + Retaliation
Scenario 1 - 65-66 Points of Rebel Superiority Fighters
5 X-wings
25 Hull, 10 Damage per Turn (TIE/In deals 6.875 Counter Damage)
TIE Fighters have 11 Hull left, X-Wings have 12(rounded) (4 Tie left, 3 X-Wings)
Next Turn - TIE Fighters have 6 Hull, X-Wings have 4 Hull left (2 TIEs left, 1 X-Wing)
Next Turn - TIE Fighters have 4 Hull, X-Wing has 0 Hull left. TWO FIGHTERS REMAIN
TIE Interceptors have 5 Hull left, X-Wings have 6 Hull. (2 Tie, 2 X-Wings)
Next Turn - TIE Ints have 2 Hull Left, X-Wings have 0 Hull. ONE INT REMAINS
It is hard to say whether Fighters or Ints have the upper hand here, the Fighters leave in better shape, but do it a turn later.
6 A-Wings
24 Hull, 9 Damage per Turn + Retaliation (Damage = # Attacking ships, more or less)
TIE Fighters have 5 Hull left, A-Wings have 10 Hull (2 TIE remain, 3 A-Wings)
Next Turn - TIE Fighters have 0 Hull, A-Wings have 6 Hull. TWO A-Wings REMAIN
TIE Ints have 1 Hull Left, A-Wings have 3 Hull (1 of each)
Next Turn - If the A-Wing attacks first the TIE dies, if the TIE goes first it will DIE on the return fire, but destroy the A-Wing with Counter. THE TWO TIE
Clearly the TIE Ints win this round, as they have a chance of winning, while the TIE Fighters are likely to fail.
So what does this mean? TIE Interceptors are better at destroying things quickly, and clearly better at combating enemy superiority fighters. TIE Fighters have better endurance and do better against enemy bombers (Including X-Wings), coming out able to fight again.
So which do you take? I suggest a mix and match. Throws the Ints at the enemy superiority fighters and leave the Fighters to handle anything else.
Obviously Heroes can mix up this, Howlrunner helps cheaper ships more, Fel is awesome for all, Mauler Mithel is also generally awesome.