Zercher or Front Squat

Sand you say?  We'll have none of that!

Honestly, do both.  For me, they both have their place in training, although if we are talking specifically caber then I would say zercher, due to the additional hip recruitment.

I'n not sure that either will correlate directly with the caber as much as you want though, and i'm sure the argument will be made that cleans/hang cleans/snatches/etc will have better carryover.  But really, nothing in the gym mimicks anything we do to a great degree imo. 

One thing we like to have for the caber, especially big ones, is strength.  We get that with both Zerchers and Fronts, as well as a lot of pulling.  There is no too strong. 

Equally, if not more important, is the timing when throwing the caber.  How do you carry the stick?  How well do you move with it?  How off is your timing when you start the pull?  Are you hips underneath like they should be?  Do you make sure not to pitch it out?