http://www.gpuzoo.com/Compare/Sapphire_Radeon_HD_7870__vs__MSI_Radeon_HD_7870_2GB_TwinFrozr_III_OC/
AND CHECK THIS ONE OUT
http://www.gpuzoo.com/Compare/MSI_Radeon_HD_7870_2GB_TwinFrozr_III_OC__vs__Sapphire_Radeon_HD_7870_GHz_OverClock_Edition/
They are very evenly matched with the overclock edition.
I love gpuzoo.com, they give you a lot of info on one page, easy to read, etc.
MSI is getting better with cards and I may try one in the future.
Check out the link and see which you will choose but all the info is in the link provided above.
Good luck in your decision!