Both are very shooty armies, although they go about it in very different ways.
Wood Elves are very cool, and are probably best described as a neutral army as far as good and evil are concerned. They just want everyone to leave the damned trees alone. Wood Elves can move and shoot without penalty, and some of their magic bows and magic arrows they have available are downright mean. Hail of Doom comes to mind-- an arrow that splits into 3d6 arrows that have some more properties I can't recall off the top of my head. Their combat units Cause Fear for the most part, and almost the entire army skirmishes so you don't need to worry too much about having to rank your models up most of the time.
Dark Elves are a bit outdated, I think, particularly when their High cousins just got a new army book. Black Magic combined with volleys of fire from repeater crossbows and supplimented by the rightly-feared elvish Repeater Bolt Throwers are downright savage, and can stop opposing armies before they even get there. Then there are mean, mean, mean units like Black Guard and Executioners to do some choppin' up close, if that's more your style. They're quite well rounded, I think-- more so than the Wood Elves, which rely pretty heavily on Shooting. Dark Elves also have some of the most powerful special characters in the game-- Malekith and Morathi are both really scary-- and Malekith is probably the only one who can win against the Dwarf king on the throne with the grudge book.
In both cases, the models are excellent, and I've always been a fan of both ranges (though my pointy ears wear white-- the High Elves have always been my favorite of the three ranges.) so you won't go wrong whichever you choose. I do think you get a better deal with the Wood Elf battallion over the Dark Elf one, though.
