Unlocked, you can buy an International One X but a AT&T one is locked to AT&T brand new. You can get the AT&T one unlocked by some services, but that's an extra step.
Used, both will cost you about the same and perform the same on Straight Talk 3G. The differences will be in memory (International = 32GB vs. AT&T w/16GB) and processor (International = Tegra 3 Quad core vs. AT&T S4 dual core). So, it's up to you as to what you want to go for.