The value is Straight Talk is a flat $45 per month before taxes- about $50 with taxes. Also ST uses more than just AT&T. You can find the carrier that the phone uses by looking at the model sticker on the side of the packing.
T= T-Mobile A= AT&T S= Sprint V= Verizon
And Metro has even shown that prepaid customers tend to upgrade MORE often than post paid customers at the rate of every 6 months.