We had a saying in our darksun group. It was "Thanks Bill".
It tended to mean that something we were using that he'd wrote would suddenly cease to make sense.
My suggestion for defilers would be to simply make defiling the 'standard' method of casting. To preserve you must select a feat (which probably gives some other, minor, benefit, like +1 on survival checks, or the like).
Further, there are a string of feats which work while defiling which enhance the power of spells. +1 caster level, instant metamagics which work when defiling etc. There will, of course, be no such thing for preservers.
That makes preserving an effort. A preserver will be down one feat. A defiler has access to some powerful feats.
It means that preserving is a genuine and voluntary limiting of your power, as it should be, as it was described in the world, and in the books.