I seen someone recommended the Xbox 360 and while I personally feel it's the best choice for my family (we have all the others), chances are, you won't be able to get your hands on one before this holiday season. If you do, you will have to go thru GREAT pain in order to get one, or over pay on eBay for it. Xbox just can't get enough out to meet demand before the holiday season.
I would have suggested an Xbox if you had an older son. Xbox is an awesome system, but it's mainly for 1st or 3rd person shooters and there are a few RPGs out now for it that get really good reviews, but I've yet to see a girl like either of them. Of course, your daughters may be different? IMO, Xbox should be your last choice.
I think there are more games on the PS2 than any other system. However, more doesn't always mean better. We own all three systems and we have the least amount of games on the PS2. I will say though, if a game is available on several platforms, I usually always buy it for the Xbox 1st because the others don't touch the system for graphics.
While someone has already mentioned that Dance, Dance Revolution is not available for the Gamecube, IMO, this is still going to be your best bet, UNLESS your older daughter likes games that most other teen girls do not. I think at times a lot of the other systems put games out there and just forget totally about the fun factor. This is one area that Nintendo shines and always has, IMO. Get them Animal Crossing and you won't hear from them for days!! It's an incredible game and I think mostly any girl would enjoy playing it. There are tons of other good ones for the system too.
I'd highly recommend you visit some online sites though and get a feel for what each system has to offer.
A few pluses here:
Xbox:
Able to plug 4 controllers in out of the box.
Awesome graphics that are better than the others.
Surround sound options and high definition.
Lots of shooter games for it and you can only play Halo on the system.
Built in hard drive so no need to buy memory cards to save games.
Can burn music to the hard drive and use that music in your games.
Has a modem so all you need is an ethernet cable (if you've got DSL or Cable internet) and a Live starter kit to go Xbox Live.
PS2:
While not a plus for many, Rockstar really supports the PS2 and you get all the GTA games 1st.
LOTS of Japanese style RPGs. Xbox gets little support here, so if you really like these types of games, this is definitely your system.
Ability to go live and it's free, but there are additional purchases you must buy 1st.
Excellent selection of games
Gamecube:
It's hard to beat Nintendo for a fun game and they develop some awesome games that you'll not get on any other system.
The $30 - $35 Wavebird (wireless controller) is tops in the field. No other wireless controller begins to come close and it's the price of a regular Xbox controller.
Cheaper price for system, BUT, Nintendo is reluctant to lower games as fast as they're lowered for the other systems.
Pretty awesome graphics for the games we have, but I attribute that more to a lot of their games being cartooney.
I'm sure there is more, but that's all I can think of right now. Good luck no matter what you decide