AT&T also has a promotion running right now, for new AT&T customers, of $100 off. In other words, if you are a new AT&T customer, you can get the Nokia Lumia 900 for free (with a two year service commitment, of course).
If you're eligible for the promotion, it makes sense to go with ordering from AT&T. If not, you might want to consider Walmart. Walmart is selling the Lumia 900 for only $49.99.