Dark eldar? Common? I /never/ seen any Dark Eldar army, even in WD....
Anyways, base your choice on this, and in this order:
1) Does the 'feel' of the army (e.g. its fluff, its background, etc.) appeal to you. It's no point playing an army just because its 'good' and you hate the background totally. So think of it like this 'What would your ideal civilization be like, if you had the power to make one?'. Then, borrow a few codexes, search on the web, ask a GW staff member to talk to you about the backgrounds of the various armies. Then make two or three choices that appeal to you, and would be most like 'Your perfect army'. For instance, I always liked the idea of the Tech-crazy peace loving Tau, so I played with them. However, I also liked the dark nature of the ruthless space marines, and shaped my current SM chapter accordingly.
2) Do you like the models/units? You wouldn't want to be caught playing with 'Fast attack selection: Bikini Babes' or something silly like that. So pick an army with models and units you like. In fact, this is what draws most gamers into the hobby. I still remember looking in the window when I was little, and going 'Devastator squad.... big guns.... COOOOOOOOL!!!!!!!'.
3) Choose an army which you can play well with. We all have our gaming styles, and if you are starting a new army, this isn't /that/ important, but if you have a second or third army, this is a MUST. I advise you go for a well rounded army for now, like Space Marines, Eldar, Chaos, Imperial Guard, so you can easily tailor your army to have a lot or not a lot of models in (yes, folks, Space Marines en masse or a crack team army of Imperial Guard is possible). Thus, you can easily tailor your army over the years to your play style. A big reason for a second army is to try out a different play style, for instance I have my SMs for elite, short ranged and close combat fights, and I have my Tau for long range firepower en-masse.
4) Are the figures easy to find? Buying Tallarn raiders or Noise Marines can be hard and expensive, such figures and armies will have to be ordered online, and cannot usually be found in-store (unless your local GW store is a good one). Also consider the range of figures and units, lots of people are attracted to an army which has recently had a codex update, for the reason that it has more units than before. More units gives more variation, and helps shape your army, and it's just fun knowing that you can have 20 types of Leman Russ or 3 variations of veteran squads, and you can field all of them on the battlefield!