Here's another good question, i'll be willing to test this with some help

what is more important life or def?

will 999 life and 600 def proportionally cause less or more dmg than 999 def and 600 life?

by "proportionally" i mean % wise, 150 dmg on 600 life (and 999 def) may be lower than 200 dmg on 999 life (with 600 def), but proportionally it is more dmg than that

150/600 = 25%
200/999 = ~20%

does anyone know the answer, or would anyone be willing to help me on my endeavor?

In combat, you get cheap-shot'd a lot and take a lot of damage regardless of your DEF. (This is particularly true when they do a high damage Tech and get a Crit.) But overall, I think you'll find the relationship of DEF/LIF to be similar to SKI/SPD. It's too close to call most of the time and you don't want either of them to be too low.

The one sure advantage that high Life gives you over a high DEF is that your monster will be able to Drill longer by getting more rolls and last longer in Errantry. DEF doesn't help anything except battle.