I\'d probably recommend taking full command, you\'ll probably need them to tip the balance in combat resolution. As for the extra rank... I suppose it depends on how big a battle you\'re playing. In less than 2k I wouldn\'t bother as they are primarily there to boost the numbers and don\'t really affect the game that much. Dark Elves are hideously expensive points-wise as it is so I\'d probably spend it elsewhere.
There are three main reasons for having an extra rank:
1: Extra rank bonus in combat resolution. This is great in theory but you only need to take one casualty to lose this benefit so it\'s not really worth it.
2: Outnumber bonus. Cavalry count as two models so you\'d have a unit strength of 20. Most of the units you\'ll be fighting will likely be larger than 20 to start with unless they\'re very good quality troops so again this is of dubious value. Of course your unit causes fear so if you can outnumber them it is a huge advantage as they will automatically break if you win the combat.
3: Casualty replacement. The big downside to only having one rank is that for every casualty you sustain you lose a number of attacks from your frontage. With a second rank these casualties can be replaced allowing the unit to stay effective longer. However Cold One Knights should be used as shock cavalry with the aim of breaking the enemy in the first round of combat. Therefore you shouldn\'t get bogged down in an ongoing combat which would make this less of an issue.
There are plenty of arguments for and against a second rank but I\'d suggest giving yourself a bit of flexibility. Get hold of some Neodymium magnets (you can get them from loads of places these days, try Ebay) and make it so you can swap the command accoutrements for lances on three of your models. That way you can either have one big unit or two smaller ones as the circumstance dictates.
