Not entirely sure how the networks work their contracts in the US but what is to stop you dumping T-Mobile and going to another operator who are carrying the phone that you want? In the UK you can switch to another provider as long as you have fulfilled the minimum term of your contract or are prepared to pay for the rest of it.
Apart from that I would think about the fact that based on the launch here in the UK, you should treat any date given with a pinch of salt right up to the point where you have the phone in your hand. Dates change, deliveries get delayed and sometimes people just make stuff up. On my original order I was given 3 different dates by 3 different people for 3 different reasons, all from the same company and all on the same day! And thats without the fourth option supplied by HTC UK themselves.
As for the S4, well again, believe it when you see it in the flesh and not before, but although it may look good on paper, that is exactly what it is designed to do, look good on paper, especially to the uninitiated. 8 cores might sound twice as good as 4 cores, but in the good old US of A you will get the speedy mobile data munching LTE/4G version of the S4, which has a (wait, better sit down!) boring old pedestrian quad core proccessor and not the super octo mega core version. Ask any professional photographer (or even an ex-pro, which would be me then!) about mega-pixels and they will tell you that it is all about size rather than quantity, and you quickly you reach a point where putting more pixels onto a small sensor (therefore making the pixels smaller) starts to have a detrimemtal effect on quality rather than improving it.
Of course we can go around the Samsung v Apple v HTC tree all day long, but the truth is that each of these devices is now far more powerful than it will ever need to be and the choice comes down to personal preference and subjective opinion. The HTC One's display for example, is actually a higer resolution per square inch than Apples legendary retina display, but as the difference is actually invisible to the human eye, does it really matter?
It sounds to me like you want the HTC but are just in too much of a rush, for me it has a bunch of unique features that I will actually make regular use of rather than some cutting edge tech that I will never use, and when I consider that I am going to be stuck with this phone for up to two years, I would hate to have to spend that time kicking myself for not waiting another week or two.