Doug,
I would think it would just depends on if you plan on
using some kind of matching/tuning system at the base of
the tower. If you do, then I'd use coax to the tower.
But you can do the same thing with ladder line, I'd just
find it easier to 'string' (actually, bury) the coax.
As for the RF 'messing' with the ladder line, it probably
won't make a lot of difference.
'Doc