What is the price difference between the laptops?
The performance difference between 6GB and 8GB is not going to be a factor.
Is the Toshiba HDD a 7200rpm model as well?
RPGs are known to favor faster CPUs but the difference 460M vs 480M should not be a factor.
NVIDIA GeForce GT 420M review
Shows the GT 420M about 12% less powerful in 3DMark 05 and 8% lower performing in 3DMark Vantage.
But Civ5 is known to heavily favor Nvidia video cards over AMD/ATI video cards.