There are currently three FW models for Dark Eldar in 40K, the Raven Strike Fighter, the Tantalus heavy transport,and the Reaper heavy support with a haywire cannon.
The current rules for all three, which are valid for 6th edition of 40K, can be found in one book, Imperial Armour Apocalypse, Second Edition. The slightly newer Aeronautica book only contains the Raven fighter.
My personal opinion on them - I would go for a Tantalus as I think it is the best looking of the three, an outstanding model, and I would really love to be able to field one. However, it is also quite expensive, the most of the three by far.
The Reaper is not a bad choice for your army, but it does have limitations weapons-wise and it dies just as quickly as any of our other vehicles. It can be easily converted from a standard Raider.
The Raven is an older gen model and not at all attractive to me. It is also primarily an anti-infantry system, of which we already have plenty. Against other flyers and vehicles, I would rather field a GW Razorwing at this point. At least until FW comes out with a new model and/or rules for the Raven.