Exactly. You can't really compare the performance between 1GB and 2GB, you can compare what you will be able to run and what you will not. Once the VRAM limit is reached, your game will slow down to a slideshow.
If you are on a budget limit or expect to be able to upgrade soon, go with 1GB. If you can't forsee any upgrade money coming your way, try to fork out some more for a 2GB card.
Take into consideration the fact that the next-gen consoles are about to come up, meaning developers will start creating more demanding engines that will carry to the PC. By then, 1GB might be less than what the consoles have.