6 post is a ramp and ball bearing
4 post is ramp only but can be pegged to allow you to use a starter if you're racing.
So, personally, unless you're racing, go for the 6 post if he's got one as although the 4 post is the newer design, and will probably need the ramps greasing between every service, although it's not too much of a hardship to do as an interim task.