Great Old One
And what does that advantage/ disadvantage represent? The prone defender being unable to properly defend themselves vs a melee attack (A), or (for ranged attacks) the difficulty in hitting a prone target from a ranged attack originating from your space (D).
Spiritual weapon is much more likely to have A (the prone defender being unable to defend itself properly vs the melee attack) than B (you having a hard time hitting something due to being far away).
Ergo A trumps D.
I bet you London to a Brick this is what the Devs would tell you as well.
And no, they don't, because once more, I'm telling you about the rules, which are clear and precise in this case. You can have whatever trump whatever in your campaign, obviously, and the devs certainly tell you this as well, but as for the rule, it's clear, simple, and unambiguous.