Uhh the MSI is slightly faster clocks than the Sapphire like 25 mhz but the Sapphires memory is a little faster than MSI by 100 mhz. I would do the MSI as memory speed is not that much of an issue. Also you can use MSI burner to overclock and such and only $15 more
i have two 280x's in cfx with very bad external exhaust. It's all blown into the case. The airflow in the case looks after that.
In average gaming it's like 65-70c, and worse case scenario high seventies. The top is card is obviously always higher temp, cause heat rises and it's doing most of the work even with 90% scaling.