[Shovah32]

go with the E9. The extra defence from water is worth almost nothing when swarmed(which happens alot to size 6 units) where-as the protection bonus and the ability to never tire make E9 extremely useful.
W9 will boost your killing power a little, but it's surviving+the cold aura that makes Niefels great.
An E9N4-9 Niefel Giant is extremely hard to take down and certainly much harder to take down than a W9N4-9 giant. In the long run, due to the survivability, the E9 giant will also probably do more damage.