The 306b can pull slightly more watts off 12v then the PL6. I've had a 306b for quite a while and have no complaints. I just got a PL6 as well but have not really gotten into customizing it yet so I won't make any decisions yet on which is better. In short pretty sure you would be happy with either one. If you want just the basic functions the 306b is great. If you want to really get into customizing things the PL6 lets you do just about anything.
The one thing I found interesting in the PL6 is the ability to set the frequency (31.25 kHz, 62.5 kHz, 125 kHz). It seems for small packs it uses a different frequency then for large packs. Have not looked into just what that means yet really. According to this thread
though it just affects the efficiency (watts in vs watts out) of the charger.