Jump on T-mobile deal or wait?

My wife and I need two smart phones by Christmas. Currently we don't have any contract cellphone plans (I got a cellphone from work and my wife has a pre-paid cellphone). T-mobile is currently having a deal with a family program for $99.98 month. It includes two phone lines, unlimited calling, unlimited texting, and unlimited data (throttled after the first 2GB). It's tempting to jump on this deal but I have no real need for the phones or the plan until after Christmas.

Should we do the following?

A. Take the deal and buy a HTC Sensation ($199) and a T-Mobile MyTouch Slide 4G ($199) to go with it.

or

B. Wait until December for a Christmas deal.

Thanks in advance! :-)