The idea of having Will points and experience points be the same sounded neat at first, but I'm not so into it after some more thought.
"Player's Option" (a sort of AD&D 2.5 edition) had character points that could be spent like Will points, and I think I did that once in an entire (multi-year) campaign. If these points can be spent on permanent improvements to the character, which may account for many successes over the campaign, then spending them to achieve a single specific success feels like wasting them. (But then, I also have a strong bias in the "permanent" direction, and make little use, in any game, of items that can be used up.)
The mechanic that Dan mentioned, of spending Will to achieve success, and thereby earning XP, is growing on me. Especially since I thought of all those ending fight scenes where the good guy gets his ass kicked for five minutes before he gets the Determined Look That Wins Combats, and mops the floor with the bad guy. (Will point spent. Level up!) It's a sort of "deciding who you are" moment, and in my personal experience*, those decisions endure.
* which unfortunately does not include climactic fights.
Wasting Away
This is a good point. I was thinking that permanent increases would be expensive, to incentivize temporary boosts, but that would likely just come across as annoying.
Let's explore other ways of earning XP that could link to specific expenditures of Will.