the 5670 is quite a bit stronger. 45fps medium settings. the 3870 will be lucky to maintain 30 at low both on a 1440/900 screen.
honestly theough there both pretty pants for bf3 if you have a decent mid to high end cpu so i would save a little more and get a 560ti. if you only have mid to low say a low end i3 or core 2 quad then the 5670 will give reasonable returns for the money but the 3870 will always be weak.