Sapphire is generally more reliable in my experience in dealing with 1000s of ATI cards since 5xxx series.
Reliable as in they have a low failure rate or reliable as in the card have little variance between them so settings for one will be similar for all.
If it's failure rate what sort of wastage do you see? Very interested in this as it could make a massive difference if mining at scale!
Cheers
Trommie