I personally prefer pathfinders over dire avengers in just about every situation:
-Against armies with heavy troops they can really cause major havoc on them taking out expensive models.
-Against most swarm armies they will cause pinning due to low leadership values. The exception here is with nids, but the pathfinders work well to take out monstrous creatures.
That just leaves the MEQ armies, traditionally the Dire Avengers are thought to be better against MEQs, but I find that the pathfinders ability to shoot every round and their longevity really makes them a better choice.
Most Dire Avengers squads will get 2-3 turns of shooting before either being wiped out or no longer having any valid targets.
-Foot Slogging DA will loost the first round, then if they are lucky get rounds 2-3 to really shoot up the enemy, round 4 they will shoot with diminished numbers, then by round 5 they are either wiped out or can’t reach anymore enemy units.
-Serpent Mounted Avengers (usually bladestorming) will zoom up and drop out of the transport and bladestorm in round 1, round 2 they can’t shoot, round 3 they bladestorm again round 4 they can’t shoot and by then their transport is either destroyed with them in it or they have been shot to pieces.
With the foot sloggers assuming a squad of 9 DAs + 1 Exarch w/ 2 shuriken cats, 2 full rounds of shooting and 1 round of 50% strength shooting you end up with 6.67 MEQ kills for 137 points spent on the squad.
With the Serpent Bladestormers assuming a squad of 9 DAs + 1 Exarch w/ 2 shuriken cats & bladestorm, 2 full rounds of bladestorming you end up with 7.39 MEQ kills for 152 points spent on the squad (I am not figuring in the cost of the transport since it is a decent tank in its own right).
With the Serpent Bladestormers and adding in a Farseer with Doom, guide, & a spirit stone assuming a squad of 9 DAs + 1 Exarch w/ 2 shuriken cats & bladestorm + farseer, 2 full rounds of bladestorming you end up with 14.43 MEQ kills for 272 points spent on the squad (I am not figuring in the cost of the transport since it is a decent tank in its own right).
With Pathfinders you can usually get them behind 4+ cover (which becomes 2+ for them) they have good range so that they can usually shoot every round, and they should last nearly the whole game due to their cover save. So I am assuming that rounds 1-2 they get their full shots then loosing 1 model per round after that.
As equivalent point costs a squad of 6 Pathfinders is 144 points which is close enough to both the first two configurations of DAs for comparison.
Round 1: 1.75 MEQ Kills
Round 2: 1.75 MEQ Kills
Round 3: 1.46 MEQ Kills
Round 4: 1.17 MEQ Kills
Round 5: 0.88 MEQ Kills
Round 6: 0.58 MEQ Kills
For a total of 7.59 MEQ Kills which beats out both squads of DAs at similar point levels.
For the farseer version at you can get 11 pathfinders for 264 points assuming the same kill ratio as the previous example you get:
Round 1: 2.90 MEQ Kills
Round 2: 2.90 MEQ Kills
Round 3: 2.61 MEQ Kills
Round 4: 2.32 MEQ Kills
Round 5: 2.03 MEQ Kills
Round 6: 1.74 MEQ Kills
For a total of 14.50 MEQ Kills Which is just above that of the Dire Avengers with the Farseer.
In both cases the numbers come out to be about the same, and yes if the Dire Avengers get to shoot every round with their full attacks they will out perform the Pathfinders every time, but at least in my experience Dire Avengers just don’t last that long. They either get charged, their transport gets blown up and they get entangled then shot up, or they get shot to death and wiped out before they can really be fully effective.
Pathfinders on the other hand are much more reliable, have more staying power, and the versatility of them being able to take out heavy infantry, pin light infantry, and kill about the equivalent of MEQs make them very worth while.