Radeon 5570 is the better choice.
The GT 430 has the exact same memory bandwidth as the Radeon 5570. However, the Radeon 5570 has 16% higher texel rate which means it is more efficient in anisotropic filtering. Furthermore, the Radeon 5570 has 86% higher pixel rate which is quite useful if you turn on anti-aliasing in games. Also, the Radeon 5570 uses around 20 watts less power than the gt 430, and you don't need extra power connectors for it.
All in all, Radeon 5570 is much faster and less power hungry compared to the gt 430.