What is better net damage or damage and why?

Net Damage is the amount of damage that will be dealt to an enemy regardless of that enemy's damage reduction. So, it is pretty much a set amount of harm that will be inflicted to any enemy regardless of how protected it is.

Net Damage increases are generally much smaller numbers than Damage increases.

Damage is the measure of how dangerous you are, but this number will be offset and reduced by the enemy's defenses.

It is safe to say that general Damage is more beneficial in early game, but Net Damage get more important as enemies build up defenses (or if you try to go up against the mega boss.)

It is tough to say for sure though, as the game has no Bestiary or indication of Might for the bad guys, so we just get to try, guess and hope.

But overall, try to keep a balance of the two with Net Damage being more important in the later rounds.