Yes the two are very similar if you just take a quick look at them. But if you take a closer look (read through both armies armybooks) you'll notice a large number of diffrences.
The base concept of each unit of the two armies are the same but the detail and special rules are very diffrent from eachother.
I'll list the things that I can think of ATM:
In general HE units gets more special rules and costs more than DE counterpart.
HE gets +1 to dispell and DE gets +1 to cast. HE magic is defencive, DE magic is offencive... it's like day and night here.
Both armies magic items are very similar to eachother except that HE items seemes to cost a little bit less for no particular reason... guess HE massproduce those so they must be cheaper right
hehe. GE still have more magic items than HE most because of the Albian treasures and the CoS-itemlist.
Dragons: Identical except breath weapon.
HE Griffon vs. DE Manticore: Identical except that griffons have higher Ld and thus costs more.
HE Great Eagles vs. DE Dark Pegasus: Eagles are better at fighting than pegasus but pegasus have better land movement than eagles. Eagles can be taken as rare choise where Pegasus can't. Pegasus costs more.
Characters: Identical to the stat but HE can tweak their characters by giving them 'Honours' like the WE Kindred system. However HE can't tweak their characters to be like the DE Beastmaster or Assassins that're very uniqe.
Archers: HE have longbows and shoots with two ranks, DE got light armour & reapeater crossbows. HE pays more cuz the extra " they get from longbow. But IMO HE archers are still somewhat inferior to DE crossbowmen.
Spearmen: HE got to fight with three ranks. DE spears gets nothing special. HE spears cost more (obiously)
Fast cav: HEs Reavers don't have any uniqe special rule and neither do DE Dark Riders, but both have elven steeds witch is great for fast cavs. DE gets the advantage of having repeater crossbows and the Reavers are special where Dark Riders are Core therefore gets the upperhand. They costs the same amount of pts.
Cavalry: HE got Dragon Princes, where DE have Cold One Knights. Cold Ones have better Ld than Dragon Princes, causes fear, gets better armour (scaly skin), but suffers from stupidity and reduced movement. Dragon Princes are limited in number, have dragon armour. DE Cold Ones costs more than Dragon Princes.
Scouts: HEs Shadow Warriors are identical as DE Shades with an exception of Shades having reapeater crossbows instad of longbows. Shades cost more than Shadow Warriors.
Chariots: HE chariots are weaker than DEs but moves faster... DE chariots causes fear. DE chariots costs more.
Reaper bolt throwers: Identical of every aspect (except the look of the models
HE Swordmasters vs. DE Executioners: These two are identical to the stat, but HE strikes at I order where DE gets killing blow. HE gets restricted in numbers too. Still HE costs more.
HE Phoenix Guard vs. DE Black Guard: These two are again identical to the stat. HE causes fear, DE has Ethernal Hatred and Stubborn. DE costs more.
There was all the units that was "like" the eachothers counterpart.
Lets se what the diffrent units are then: (these are my oppinions
Silverhelms: like a med cav. with reasonable cost and core... gotta love these!
Lothren Sea Guard: like a combined archer and spearman. Costs loads of pts and are restricted in numbers... I kinda don't see any point in these.
White Lions: An somewhat OK unit to have your general in (if you want a general on fot that is) cause of the Bodyguard rule. They move like WE in woods... to bad they costs a little to much and are rare.
Corsairs: Kind of opposit of spears (spears deffencive, corsairs offencive), have neat Sv against shooting and good attacks and not costing too much... great semi elite unit. Too bad they're metal models and costs a fortune to buy.
Witch Elves: Crazy sadistic frenzied she elves... who don't love these
. Ok they've poisoned attacks, frenzy and a good attack count. They can tear up badly armoured units like a hot knife on butter. They've no armour and that makes them get killed far to easy by shooting units... hide them well.
Harpies: Nice flying unit... identical stat wise as Chaos fury (but not daemonic). Good for poking enemy units and marchblock... too bad they're restricted in numbers.
War Hydra: Wow these have almost the same stats as a dragon... almost
, they're like overweight dragons (that means they don't fly). But they're controlled by beastmaster apprentices. Costs lots of pts... but still acceptable.
Cauldron of Blood: Unitboosting static unit... like dwarves anvil of doom or TKs casket of souls. I've had bad experience with these... but others have won battles because of these.
Ok I really hope that I didn't broke any LO rules now.
that was all I think... so as you can see these two armies are somewhat diffrent in many ways. Take your pick! But I bet that if you ask witch arny you should choose you'll get overwhelm by "go for DE" answers here and "go for HE" answers over at the HE section of this forum.
I can say why I pick DE:
1. Because I love Dark Elves... not just in the world of warhammer but in just about every other fantasy universe as well ^^.
2. Their models was the most good looking at the time I took up WHFB again IMO.
3. That they were one of the most challeging armies to play.
Good luck to you!