If you had to take one of gross spent or net spent and decide, on it's own, which one is a more straightforward indicator it has to be gross spend. How much are you spending on improving or maintaining the quality of your squad? That much? Okay.
Net spend you have to break down transfer by transfer and decide if the player going a) matters b) was "replaced" adequately - either by quality in the position left or strengthening elsewhere and c) was under or overvalued when sold. It's too subjective to really mean much imo.
Although in regards to (b) above, I actually disagree that players are "replaced" with transfers, one for one, as though they were simple mechanical parts and that were even possible. Especially in terms of important players. We obviously lost Carroll and it changed the way we played - we didn't really "replace" him with Kuqi. Man United didn't really replace CRonaldo with Valencia or Obertan, the team took a different shape and adapted.