XFX has a double lifetime warranty (so you can transfer the warranty once to someone else if you sell it).  They will also accept warranties if you use the OC position.  Sapphire will also warranty it if you use the OC position but the warranty is only one or two years.  From what I've heard, all other brands might void the warranty if you put them in the OC position.

I think HIS will give you the most amount of accessories (crossfire connector, PCI-e adapters, DisplayPort converters, etc).  MSI will include covers to protect the connectors.  XFX includes the least amount of accessories.