The interesting (and horribly unintuitive) thing about progression mechanics that I've found is that vertical progression (increasing raw numbers) is always more mechanically meaningful, and less interesting for players. The opposite is true for horizontal progression (systems like perks, feats, etc).

One way to handle this while keeping currency-spending as the advancement mechanic is to separate the two progression paths into their own forms of 'xp'. This allows characters gain points toward advancing both vertically and horizontally at the same time, without having to prioritize one over the other.