I've had Sapphire GPU's in the past and always found to be reliable. As I've never heard of VTX is the Sapphire worth the extra? The VTX does look great value.
VTX3D HD 7870
https://www.overclockers.co.uk/showproduct.php?prodid=GX-013-VX
Sapphire HD 7870
https://www.overclockers.co.uk/showproduct.php?prodid=GX-314-SP&groupid=701&catid=56&subcat=411
VTX3D HD 7870
https://www.overclockers.co.uk/showproduct.php?prodid=GX-013-VX
Sapphire HD 7870
https://www.overclockers.co.uk/showproduct.php?prodid=GX-314-SP&groupid=701&catid=56&subcat=411