Should I buy the asus dual 9060XT 16GB version for $400 or the 8GB version for $305?

If you can afford it, spend the extra money for 16GB model. The 16GB of VRAM may not be essential for the games you play right now but, unless you plan on upgrading your GPU again in just a couple years, you may find you want the extra VRAM for games releasing in the future. It's always better to have the extra headroom and not need it than need it and not have it. An extra $100 now gives you that peace of mind and should provide a little extra longevity to the card's viability. I'd say it's a worthy investment, especially given there's no telling what future GPU pricing will look like.