VisionTek has a lifetime warranty and very good customer service. Heard nothing but good things about them.
Sapphire has a 1 year warranty and sucky customer service. I've heard many reports about them not honoring their rebate process too.
I went with VisionTek this time around, but haven't used their customer service or the lifetime warranty yet. I would definitely go with VisionTek.
Keep in mind that most of the cards that are manufactured generally have identical or at least similar designs, so there is no better or worse chance that the card will die if you go with a "good" company or "bad" company... but if it does... then customer service and warranty makes all the difference. It's good to be safe just in case. Ask yourself if the slight premium of $5-$10 is worth it for the insurance.