Sapphire's HD 7950 Vapor-X comes with a large dual-fan cooler that uses the company's famous vapor-chamber technology. On the Vapor-X you will also find a "Lethal Boost" button which switches to a second BIOS with higher clock speeds. With this BIOS, clocks are increased to 950 MHz GPU and 1250 MHz memory.Sapphire's HD 7950 Flex uses a higher GPU clock than the AMD reference design, which will help gain some extra performance. Memory clock has remained the same. Flex Edition cards come with a unique feature. The card can support three DVI/HDMI monitors without the need for an active DisplayPort adapter. This makes life easier for users who plan to use EyeFinity.If it were me i go for the Vapor-X it's up to you.
These three I'd have no issue buying. And they are all cheaper. The Gigabyte I own and as far as boost(none of the 3 have it) my Gigabyte will hit the boost clocks easily, and it's already clocked within 25-50mhz of the boost clock speeds.