I returned a HIS HD6790 that quit for indeterminate reasons. The RMA process was not difficult, and I got a working card back. Keep in mind that almost all RMA processing is done by third-party companies, and any of them may have issues.
If you are playing on a single 1920x1080 monitor, 2GB should be sufficient, so I might choose MSI in that case. HIS is decent, imho, but I like MSI for video cards just a little more. If you like to overclock, I'd expect better results from MSI. Are the clock speeds the same on the two cards?