12 point for chrome sockets and wrenches. If you end up needing to use a breaker bar, only needing 30 degrees swing instead of 60 is nice.
6 point for impact sockets. The only thing that would convince me to have a 12 point impact socket would be if I were likely to encounter a 12 point fastener which I would need to run with an impact. In my line of work, it happens so very rarely that I just put the chrome socket on an impact extension (but that's running on the verge of provoking a whole new "oh not this again" thread.
Most rusted/damaged fasteners that will be rounded by a 12 point will also be rounded by a 6 point. There a very narrow margin of fastener damage, in which one can be damaged enough where a 12 point will round it, but a 6 point will remove it. It is a very narrow margin. So much so, that in my opinion, it's not worth keeping another set of sockets for it, although that's a matter of opinion, and some will reach different conclusions.
Try them both, form your own opinion based on your own observations, and if anyone tells you the opinion you've reached is wrong for you, tell 'em to go pound sand.