Best to compare the specific cards in the benchmarks that are meaningful to your uses -- Tom's has a very large collection of graphics reviews and comparisons.
It is that simple.
A card with 1GB of ddr5 is going to be much faster than the same card with 2gb of ddr3.
If you're taking about a card based on the same GPU (like a 5770/6770 or a 5670) then it's better to go with the GDDR5. If you're talking about different cards then it depends on exactly which cards you're talking about.