Honestly both are feeble cards but I would go with the GDDR5 due to having double the effective bandwidth than the other. It isn't about if it can use the extra memory but it is about how fast can it get data to and from the gpu. The 5570 is about the best that you can afford. Performance wise they are ok but don't expect to much of it.
I'd say it's enough to pay $10 more.
I was wondering if it would perform better because it has half as much RAM.