What is your resolution? If you play on a 1080p monitor, you won't have to get the 4GB one. But, since these cards are sold at the same price, you might wanna get the 4GB one, just in case you need that extra VRAM in some games. You should also consider that some Sapphire cards can get a ted bit noisy during heavy use. The ASUS card is almost silent. It is also clocked higher than the Sapphire version.
Conclusion: If you play on a 1080p monitor, get the ASUS version. If you play on a bigger resolution, get the Sapphire one. Don't listen to those people who say that more VRAM will make your rig future proof, because it won't.