This kind of trade-off spell could work better for fullprepared casters
As a bard, you already have very limited spell choices. Because of this, it will be much more difficult to make "tradeoff" versions of spell work, since you usually can't have both of them at the ready to make the choice of which one to use depending on the situation. However, I could definitely see this concept for a spell working on a full"prepared" caster.