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So, I have been toying with some ideas and I am not sure which would be the better route.m(I am including the pros/cons that I forsee). First off I WANT psychic powers in the army hence the limit choices. In addition, I am only posting the two armies that I would add later to be my next army.

Eldar
Pros
Lots of varied, specialized units
Wonderful Pyschic powers
Lower costs for units
Speedy
Battle Brother

Cons
Just as fragile as Tau
Basic units are not really any better than the Tau (and until the new codex)
Some of the units have similar capabilities and not for the best point comparison

Grey Knights
Pros
Psyjer, Psykers, everywhere
Extremely tough, assault units

Cons
Dear god, the cost/Not many models for the points invested

So DakkaDakka, what is the thought?

I would go eldar. They are really good for going at the enemy while your tau sit back and light them up. Eldar are nice and quick.

Eldar because they have psychic defense that helps protect the tau. While grey Knights also have very good psychic defense, theirs really only helps them. Tau on their own don't really have anything.

 prophet102 wrote:
I would go eldar. They are really good for going at the enemy while your tau sit back and light them up. Eldar are nice and quick.

And that was my natural inclination. GKs are nice, but the Edlar are coming out with new stuff soon (hopeflly) and I like themodels...

Eldar, just to promote more xenos.

Eldar—They give Tau psychic support and speed without changing the fighting style.

Eldar. Space Marines should not ally with vile xenos, no matter what that stupid ally chart says.

Eldar. Battle brothers, so they can use their fancy psychic stuff to help you unlike grey knights.

Plus Eldrad.

I'd literally take just Eldrad and some rangers.

Personally, I vote for Eldar. GK do have merits, but it really depends on your army playstyle.

Grey Knights can provide a static gunline Tau list with an assault deterrent, making the enemy select less optimal angles of attack so they can charge squishy Fire Warriors and not nasty Grey Knights. This could even buy you another round of shooting. Grey Knights are also a perfect weapon for countering MC's, with the prevalence of force weapons in them. They are also a hard counter against Chaos Demons, who are an army who I would normally be very afraid of with Tau. If you're running mech Tau, including some Strike Squads in Razorbacks can give you some much needed anti-tank firepower, you can run 2 Ionheads and a Railhead + Longstrike and not worry so much about AV14. The best reason to take Grey Knights is the units resilience, they have the ability to seize forward objectives by force, something Tau troops can't do. That said, I'd probably take Marines for cheaper if you're only taking them as midfield units.

Eldar sync perfectly with Mech Tau and Crisis Suits. The prevalence of Lance weapons can really hurt enemy AV14, moreso than an additional BS3 Railhead could. The large numbers of fleet units can really assist you if you have an aggressive playstyle, you could use a unit of striking scorpions to tie down a larger enemy unit for long enough to allow your Crisis Suits to strike at a target that would otherwise have put them in charge range. Warp Spiders can sync with Crisis Suits. Some warlocks on jetbikes with a Farseer could work well with a Farsight Deathstar. These things said, the Eldar aren't really going to provide Tau with troop options that really allow them to take enemy objectives. You're still going to need to shoot them off, or use elites to assault them off; if you want 20 man units like the Guardians can give, you have Kroot. Just something to think about. You could always take a squad of Wraithguards for chuckles, though. I don't feel, however, that Eldar would work well with a gunline Tau list, since most of the non-heavy weapons are too short ranged to work very well with the Pulse Rifle.

Who would be a better fitting ally for Xenos than the military arm of Inquisition's Ordo Xenos
Almost as good as Harlequins allying with Chaos

It also depends on what type of longevity you'd like from your allied contingent. It is rumored Eldar will be next receiving a new Codex in June. This time next month, we may even be getting glimpses of the new models.

However, Eldar do have some nice synergies. Wraithguard with a Crisis leader giving the WG Hit and Run would be nice.

Jetbikes add a great mobile scoring unit, although three man units are quite fragile and a single Helldrake erases them.

3 word:

RUNES OF WARDING

and I swear, Warwalkers are the most effective anti-air I've seen even without skyfire.