6850 for general performance and dependability but drivers are something to be desired. The 550ti is overpriced and weak compared to other card but it has features such as physx and cuda as well much better drivers.
90% of the time that is rarely the case and personally have had good luck for years without being burned by issues. However they do happen especially for those who update to the latest version expecting gains, that is when people get burned by that once in a while version that gets every ones attention with rather exception results. As for ati drivers it is common place for people to search for different versions that provided desired results instead of using the latest or the version they were using prior. As for me I don't play every thing out there and more conservative hell I have gotten by for almost a year without needing to update.
I will next month now when I overhaul my rigs and new upgrades.