Assuming Apple has not done something remarkable like release an overclocked part, we already have a good idea that the ATI card will be 15-20% faster in most games than the GT130.
Now personally, I think that ATI support and drivers have been more stable and I think that ATI silicon has fewer issues with failing compared to Nvidia, so if I end up with a 2009 iMac it will be the ATI equipped one even if it hardly benches any better than the GT130.