What is the difference between the Chaos and Confusion spells, other than spell level? Or is that all, the ability to cast it as a fourth or fifth level spell?

By the way:

I know that lower THAC0, AC and saving throws are better.
When a spells is described as "the target get a saving throw vs X -2 to avoid the effect",
is the spell easier or harder to resist than a spell without modyfier?

if you're looking to affect the enemy then having them make a save at minus a larger number is better (+ modifiers make it easier to save, - modifiers make it harder) but it's also dependent on their stats.
So a saving throw at -2 means it's easier to resist the spell than -4 but harder than no modifier, and +2 is harder to resist than +4 (and no modifier is like +/- 0)

thanks