There is only ONE rule for deciding keys. SOUND.
If it's a vocal - the voice has to sound best.
If it's an instrumental - the melody instrument has to sound best.
That's it. Period. End of story.
Make it the best for the instrument that carries the tune.

As a singer, I make sure my keys suit my range and comfort zone before anything else. Boo (or any sax player) would have an awful time accompaning me because I often play in B, E, or F#. Can't be helped - sometimes, that where my voice NEEDS to be.
I hate it when people tell me to play something in "the original" key. I have no idea what that is most times ! I know it in the key I LEARNED it in, because that's why I learned it in the first place. I learned it to perform it myself, not accompany others in someone else's key.
That's the biggest reason I discourage amature guest singers. Most don't know the key, the words or the way to sell the tune. They're just having fun. They can do that at home. When I'm working ..... I'm too busy to babysit Karaoke pipe dreams.

On a brighter point: I'm almost moved into my new birthday house ! Yipeeeeeeee !