I go with Hugo since he makes the most sense to me story-wise. Although Geddoe is a particularly badass Flame Champion and has some rockin speeches.
In terms of mechanics, Geddoe is better with better with both True Runes. Hugo has only a B affinity with both while Geddoe is a B+ in Fire and an A in Lightning.
To be perfectly honest, I much prefer Lightning over Fire any day. The spells are stronger and they have no chance to fry your own party. Yes, they hit all targets, but that is unnecessary in most fights. Geddoe can easily reach S rank in Lightning with a Wizard Hat and one Lightning Magic Ring, but would require 2 rings to get there with Fire. Hugo would require 3 rings to get to S in either. Hugo is also much better suited as a fighter with his high speed and A ranks in Swing and Heavy Damage.
I say make Hugo your Flame Champion but don't bother using the Rune. Spec him out as a fighter and let Geddoe destroy things with Lightning Magic.