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I have a Tau army and am adding some Eldar allies to it. So would a Falcon or a Fireprism be a better addition to a typical Tau army?

The HQ would be a Farseer and Troops would be either Pathfinders or Jetbikes.

And one more question, are Warspiders a good choice for a FA option?

The Falcon is handy for transporting small squads, like Fire Dragons. The weapons it gets aren't great, but good enough.
Fireprisms work best in pairs, so there goes that idea. They're nice for taking out squads, though. The beam is handy as a backup weapon against vehicles.

Warp Spiders are fairly solid, but still aren't much use in CC. Their S6 Assault 2 guns are short range, but they can jump into cover after firing (usually).

If it was me, I'd choose the Warp Spiders and GJB squad, and chase them around with the Farseer in the Falcon.
Most Eldar buff powers don't need LoS, and can be cast from any point of the Farseer's transport vehicle.

I am also running tau and thinking about getting eldar allies until i can get enough for a full eldar army.

from what i have seen on fire prism tactics they are usually ran best in pairs so a falcon may be more worth it for the different choices in weaponry but i have not looked a lot into eldar yet so i may be completely wrong on that.

as for warpspiders they are one of my favorite units for eldar so i know i will be taking those and i have seen them in alot of eldar lists on here

anyway i am also interested in peoples opinions on this along with the OP

The Fire Prism is great for Tau, even if you only have one.
That lascannon blast is awesome at mulching marines, bikes, terminators... basically all the marine stuff that we would otherwise have to pay special attention to with battlesuit plasma.
It also works really well with hammerheads, because it has the same choice between anti-tank and anti-infantry shots. The only problem is that it can't use markerlights.
EDIT: I have played several games with one and I've loved every second of it. Of course I only use one suit team.

The thing that upset me is that in the RB it says that if a power does not specify anything, then it requires LOS. So no shooting divination powers out of a wave serpent.

 DakkaHammer wrote:
The thing that upset me is that in the RB it says that if a power does not specify anything, then it requires LOS. So no shooting divination powers out of a wave serpent.

Yup, only Eldar codex powers can be used out of a closed transport.
So, Fortune, Guide and Doom all work, BRB powers and the rest, nope.