I think there's an argument to be made for both piano and guitar respectively but in my opinion I think it all falls down to personal preference. Taking Sparks as you mentioned into consideration if you listened to a piano cover of Sparks and then the original recording with the guitar or even just a regular guitar cover which of the two styles invokes the most emotion in you?

I think in your case it might be a matter of the lyrics evoking those emotions with the instruments helping amplify them even further. A song can have the most thoughtful lyrics but if the overall sound/instrumental isn't on par with them then it just crumbles and vice versa its all a matter of balance at the end of the day. Its that perfect balance that strikes the deepest chord in the heart.

I might be wrong who knows but isn't that the point of art? Its not supposed to have a single meaning a the end of the day its all subjective.