Well, I have both, they are the last two codex's, and are all the rage. Make an appearance at the local flgs, and just count how many of the first I was marine/ then vulkan/ then space wolf/ then blood angel/ then grey knight players there are. Within a year they will more than likely be Templar. They find something that is really cool about the new codex, and there you have it, that is what they are playing with their grey army.
I have no issue with these players, in fact, they do us quite a good service. That is, they help us learn and adapt to the latest "in" army. Not only that, but they are having a tremendous amount of fun, which is good for the hobby. So yes all you new players, go marines, and adapt your army until you find something you really like. Then settle down please, and choose a codex.
So now that the Dark Eldar are no longer the "latest" thing, how many of players of Dark Eldar are there. Well, a good number of them are Craftworld Eldar players that do not like their codex and are waiting.
So in reality Dark Eldar numbers are dwindling as we speak. (the same can be said for Blood Angels). I still enjoy seeing and talking to people that are dark eldar players, so I think there will always be Dark Eldar players for some time now. The problem with it of course, is it is a hard codex.
I think the following codex's are hard. I mean they are hard to learn the ins and outs to come up with a very solid list.
Tyranids (were did they all go?)
Amongst the easy codex's to play.
I really think Space Wolves fall into the middle ground. Nice complex codex, with excellent options and flavor.
The next codex's, where will they fall? Necrons, Tau, Sisters of Battle, Legions of Chaos, and Black Templar?
This topic came up, mostly because it literally took less than a day with the codex in hand, to put together a strong Grey Knight list. Of course it needs refined, but I really like the list. Dark Eldar on the other hand, I am still working hard to refine, or you could say it took me from release date until a few months back to really figure out what I want. In fact, I need to go pick up some of my last few models for my Ard Boyz 2500pt Dark Eldar list this week.
A lot of this may just be me. I believe that the codex's of 5th edition are the best out there. I also believe that they are pretty equal in terms of strength and competitiveness. It could also be, that certain armies just fit my play style.
Either way, ATM I am an Imperial Guard player, a Blood Angel player, a Dark Eldar player, and finally, I will end up being a Grey Knight Inquisition player, once the rage slows down a little.