Jerry Tank Panniers - Vertical or Horizontal?

I'm going to be building Jerry-Tank panniers.
Cheap. Super-strong. 20L per side. Rounded corners! Any colour I want! Cheap!
And available in every city, every town, in case I actually manage to bin one!

Question: I can install them vertically or horizontally:

Vertical-PRO's:
lower center-of-gravity
smaller dust seal area
more out of the way of pillion
looks more like traditional panniers

Horizontal PRO's
higher ground/object clearance
better access to pannier
may look better

WHAT TO DO???